Margot Kidder, best known for playing reporter Lois Lane opposite Christopher Reeve in the "Superman" film franchise of the late 1970s and early '80s, has died at her Montana home. She was 69.

Kidder's death was first reported by TMZ. The Franzen Davis funeral home in Livingston, Montana, said the actress and activist died Sunday at her home, and her agent later confirmed it.

A cause of death was not given.

Born Margaret Ruth Kidder in Yellowknife, Canada, Kidder got her start in low-budget Canadian movies and TV shows before landing a role as Gene Wilder's love interest in the 1970 cult comedy “Quackser Fortune Has a Cousin in the Bronx.”

Kidder later starred in the 1973 Brian DePalma thriller “Sisters,” appeared with Robert Redford in “The Great Waldo Pepper” in 1975, and anchored the 1979 horror movie "The Amityville Horror.”

But her breakout role came in 1978 as the dogged reporter for the fictitious Daily Planet newspaper in "Superman," which starred Reeve as the titular man of steel.

"I’d never read comics, so I didn’t know much about Superman," Kidder recalled in a 2009 interview with The A.V. Club. "But I read this very funny script, and I went in and did a couple scenes, and next thing I knew, I was being flown to England to screen-test, and that was that."

Kidder reprised the role three more times. And she and Reeves remained friends until he died in 2004.

The Reeve Family and Foundation are deeply saddened by the loss of Margot Kidder. Fly high, our friend, shine bright. #Superman pic.twitter.com/EYWWevThQN — Christopher & Dana Reeve Foundation (@ReeveFoundation) May 14, 2018

"When you're strapped to someone hanging from the ceiling for months and months, you get pretty darned close," Kidder told CBS. "He was such a huge part of my life. ... He was complicated, very smart, really smart, and he knew he'd done something meaningful. He was very aware of that and very happy with that role."

"Superman" was blockbuster hit and turned Kidder and Reeves into stars.

But behind the scenes, Kidder battled bipolar disorder — a private struggle that became public when she disappeared for four days in 1996 before she was found on a porch not far from the lot where "Superman" was filmed and taken in for psychiatric treatment.

Kidder later explained that the chain of events that resulted in her breakdown began when she lost the computer she had been using to write a memoir. Then, believing that her first husband, writer Thomas McGuane, was going to kill her, she cut off her hair and pulled out some of her teeth so she could not be identified.

After her story came out, Kidder became an outspoken proponent for mental health reform. And after she became a U.S. citizen in 2005, Kidder became an active supporter of Democratic and liberal causes.

Margot Kidder in 1985. Harry Langdon / Getty Images

Among other things, Kidder actively campaigned for Jesse Jackson when he ran for president in 1984.

She and McGuane had a daughter, Maggie, in 1976. She married actor John Heard in 1979, but the union lasted just six days. In 1983 she married French movie director Philippe de Broca. That marriage ended in divorce a year later.

In her last years, Kidder lived alone in her log cabin with her dogs.

In 2009, Kidder correctly predicted that she would be remembered most for playing Lois Lane as she reflected on a career that included more than 60 movies and dozens of TV appearances in an interview with the A.V. Club.

"I’ll have that inscribed on my damn grave," she said. "I still get stopped for being Lois Lane, and I’m 60 and have two grandchildren. So it’s kind of weird."