Complex sentence: Fink added as much as $1 billion to First Boston's bottom line, and was successful at the bank up until 1986, when his department lost $100 million due to his incorrect prediction about where interest rates were headed.
Split into simpler sentences: Fink added as much as $1 billion to First Boston's bottom line. He was successful at the bank until 1986, when his department lost $100 million due to his incorrect prediction about where interest rates were headed.