Securities - Narrative (Details) $ in Billions	Dec. 31, 2018 USD ($)
FNMA investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	$ 13.3
Securities, fair value	12.8
FHLMC investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	9.6
Securities, fair value	$ 9.3