Carly Simon details Jackie Kennedy friendship: 'She just loved you unconditionally'

Carly Simon is revealing intimate details about her relationship with Jackie Kennedy Onassis.

Simon shared that Onassis was like the mother she never had during an interview with NBC News for her new book, "Touched by the Sun: My Friendship with Jackie."

The "You're So Vain" singer said when she entered a rehab facility after battling addiction where she was allowed one phone call a day, she said she would call Kennedy.

"She was being the mother that I never had. And she was not judging me," Simon told NBC News in an interview published Tuesday. "She was just so supportive and funny at the same time."

When Simon won an Academy Award for her song "Let the River Run" in 1988, she said her mother congratulated her, but in the same breath noted there were other people more deserving. When asked about the difference in Kennedy's reaction, Simon said it was all love.

"She just loved you unconditionally," Simon said.

"She gave me advice like nobody else did,” Simon said. “Other people would be too nervous to tell me what they really thought about certain things ... But Jackie was forthright."

Kennedy was also known for being a bit of a prankster, Simon said as she recalled a particular instance when the former first lady fooled her into thinking opera singer Placido Domingo wanted her to be his Valentine.

She said that she went with Kennedy to a recording session with Domingo once and after Simon raved about how great he was, she received a letter the next morning from the opera singer – or so she thought.

"Darling Carlita, please be my Valentine,” the note read. “You are so lovely. I loved singing with you."

Simon said she called Kennedy to tell her the great news, only to learn that Jackie was the one who sent the letter: "Carly, did you really think it was from Placido?"

Despite their close friendship, Simon admitted she was nervous about giving people a window into Kennedy's personal life, even though she's been dead for 25 years now.

"It doesn't seem that way," Simon said. "It seems as if she's living inside me saying 'No Carly, don't tell!"

This article originally appeared on USA TODAY: Carly Simon says Jackie Kennedy was the mother she never had