Securities - Narrative (Details) $ in Billions	Jun. 30, 2019 USD ($)
FNMA investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	$ 13.6
Securities, fair value	13.6
FHLMC investments
Debt Securities Exceeding Ten Percent of Stockholders Equity [Line Items]
Securities, amortized cost	9.5
Securities, fair value	$ 9.5