Securities - Narrative (Details) $ in Billions	Mar. 31, 2019 USD ($)
FNMA investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	$ 14.2
Securities, fair value	13.9
FHLMC investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	10.3
Securities, fair value	$ 10.2