Securities - Narrative (Details) $ in Billions	Sep. 30, 2019 USD ($)
FNMA investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	$ 16.5
Securities, fair value	16.6
FHLMC investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	12.4
Securities, fair value	$ 12.4