Investment Securities - Narrative (Details) $ in Millions	Mar. 31, 2020 USD ($)
FNMA investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	$ 15,400
Securities, fair value	15,900
FHLMC investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	11,300
Securities, fair value	$ 11,700