In the next few days, Won't You Be My Neighbor? will cross the $20 million threshold at the U.S. box office, a coveted milestone reached by only a dozen or so other documentaries before director Morgan Neville's film about the beloved children's television host Fred Rogers hit the big screen in June.

Won't You Be My Neighbor?'s list of accomplishments is sizable: It has become the top-grossing biographical doc of all time domestically after passing the $8.4 million grossed by Amy, the 2015 film chronicling the life and death of Amy Winehouse. It is also the top-earning documentary in the past five years and the 14th biggest of all time, including big-studio nature movies, not adjusting for inflation.

Neville's film leads the boom that has turned docs into the stars of the 2018 summer box office, including films about U.S. Supreme Court Justice Ruth Bader Ginsburg and identical triplets separated as part of a nature-versus-nurture experiment.

"A year ago, it seems like you couldn't pay people to go see documentaries in theaters. There were a bunch of big acquisitions at Sundance in 2017, but nothing did so well. "People thought the market for documentaries was extinct," Neville tells The Hollywood Reporter

The top-grossing doc last year — excluding Disney's family-friendly nature doc Born in China ($13.9 million) — was I Am Not Your Negro, Raoul Peck's portrait of author James Baldwin, which attracted $7.1 million. Al Gore's An Inconvenient Sequel: Truth to Power topped out at just $3.7 million domestically despite the backing of a major Hollywood studio, Paramount. A decade earlier, Paramount won the Oscar for best documentary feature for Gore's An Inconvenient Truth, which grossed $24.1 million in summer 2006 and put the former vice president back on the map. 

It almost looks like the most-talked-about docs are migrating away from the multiplex, since the last two films to win the Oscar for best feature documentary were seen primarily on the small screen: Netflix's Icarus and ESPN's O.J.: Made in America.

"We thought that if our film did anything close to $10 million, it would be a home run," says Neville, who won the Oscar for best documentary for his 2013 film 20 Feet from Stardom, which topped out at $4.9 million domestically.

RBG, Betsy West and Julie Cohen's film about Ruth Bader Ginsburg, has likewise found box-office justice, earning $13.1 million to date for Magnolia and Participant Media as it winds down its theatrical run. The film staged its world premiere in January at Sundance, where both Won't You Be My Neighbor? and Three Identical Strangers, Tim Wardle's stranger-than-fiction account of three triplets who are reunited, also debuted.

Strangers has earned more than $5 million in its first month for the filmmakers and indie distributor Neon. The doc recounts the harrowing story of Robert Shafran, Edward Galland and David Kellman, triplets who were adopted by separate families as part of a scientific experiment. The three brothers were unaware of each other's existence until a random encounter brought them together.

On Thursday, Universal, parent company of Focus, announced its home entertainment plan for Won't You Be My Neighbor?. The film will be released digitally on Aug. 21, followed by DVD/Blu-ray on Sept. 4.

The same day, Participant and Magnolia announced that RBG will become available digitally on Aug. 3. The DVD/Blu-ray release date is Aug. 8.

Box-office observers believe Won't You Be My Neighbor? could gross $22 million-$25 million by the end of its box-office run, on par with such hits as Michael Moore's 2012 Oscar-winner Bowling for Columbine ($21.6 million) when adjusted for inflation.  But they note that instead of being political broadsides like Bowling, both Won't You Be My Neighbor? and RBG offer positive messages in a time of tumult.

"Won't You Be My Neighbor? has become a phenomenon. It's cathartic," says Focus president of distribution Lisa Bunnell.

Adds Neville, "Documentaries in general are telling the types of complex adult stories that aren’t getting told in Hollywood very often anymore. They are contagious."

Not all summer docs are scoring huge numbers, though. Kevin Macdonald's Whitney, which looks at the turbulent life and tragic death of Whitney Houston, has earned a more modest $2.8 million to date for Roadside Attractions, while Pope Francis: A Man of His Word topped out at $1.8 million.

In summer 2004, Moore's documentary Fahrenheit 9/11 made box-office history when it grossed $119 million to become the top-grossing doc of all time domestically, a record it still holds. The following June, another doc made a big splash when Warner Independent's March of the Penguins waddled to an impressive $77.4 million to become the second-biggest doc domestically.

In the intervening years, only a smattering of docs have been able to break out, and most were either concert pics or political treatises from Moore or his conservative arch-nemesis, Dinesh D'Souza. The two will soon be battling it out again at the box office. D'Souza's Death of a Nation, which is designed to draw parallels between Abraham Lincoln and Donald Trump, opens nationwide Aug. 3. And Moore's latest, Fahrenheit 11/9, sure to take a much more critical look at Trump, is set for release on Sept. 21.

For the moment, though, moviegoers are opting to embrace the uplifting messages offered by Mr. Rogers. Says Neville, "Anybody who says they aren't surprised how how well our film has done is lying."