The actress reportedly passed away at her home in Montana. No cause of death has been confirmed.
Kidder starred with Christopher Reeve in four Superman films from 1978-87 as the caped crusader's love interest, Lois. She also shared the screen with Peter O'Toole in a 1983 TV production of George Bernard Shaw's Pygmalion, and would later appear in the TV Superman prequel Smallville. In 2002, Kidd performed on Broadway in a production of The Vagina Monologues.
She struggled with bipolar disorder for many years, after being diagnosed with the condition after a manic episode in 1996, in which she was found in a disorientated state, having apparently been living outdoors and sheltering in bushes in Los Angeles for several days. She later used her public profile to raise awareness about the condition, and became an advocate for alternative medicine.