Securities - Narrative (Details) $ in Billions	Sep. 30, 2018 USD ($)
FNMA investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	$ 13.6
Securities, fair value	12.9
FHLMC investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	9.9
Securities, fair value	$ 9.4