Susan Buffett reveals the surprising way she found out her dad Warren Buffett was rich

Susan Buffett didn't find out her father, billionaire Warren Buffett, was rich until her early 20s. And she definitely didn't hear it from her parents.

"I didn't really have any idea about any money until I was long out of high school and out of Omaha," Susan recently told Business Insider. "There was an article in the Wall Street Journal at one point when I was probably about 22 or 23, I'd say. That was the first time I was like, 'Oh, there's more going on than I knew about!'"

The founder of children's charity the Sherwood Foundation, Susan takes part in a portrait of her father, HBO's "Becoming Warren Buffett," which premiered Monday. In the documentary, Susan provides a lot of intimate details about her parents, growing up in the family's home in Omaha, Nebraska, and what drove her father to give away a large chunk of his fortune to charitable causes.

As for finding out that her father was rich from a news article, she attributes it to her modest upbringing.

"When we were growing up he didn't have a lot of money and he wasn't famous," she said. "So we didn't see anything but a regular, normal situation in our house. He went to the office every day, he came home, he was at the dinner table. He did go upstairs and read the rest of the evening, but he was home. And we lived pretty much like everybody else. Nothing was too different. Really, we lived in what I guess I would describe as an upper-middle-class neighborhood. He still lives in the same house today."

There were certainly signs that her father was doing well, such as when her mother started a scholarship program when Susan was in high school.

"It was only a couple kids a year, but she started as soon as there was some money there to give away," she said.

After thinking on it more, Susan said there was one visit that gave her an early clue that her father was doing pretty well. Adam Smith, who wrote the pivotal book on the financial industry, "Supermoney," came knocking.

"I may have been a senior in high school then," Susan recalled. "And I remember Adam Smith came to visit our house in Omaha. And I remember when I heard the name of the book was 'Supermoney,' I was like, 'Okay, that's something' [laughs]. But I still don't really think we talked about it."