Securities - Narrative (Details) $ in Billions	Jun. 30, 2018 USD ($)
FNMA investments
Debt Securities, Available-for-sale [Line Items]
Securities, amortized cost	$ 14.1
Securities, fair value	13.5
FHLMC investments
Debt Securities, Available-for-sale [Line Items]
Securities, amortized cost	10.2
Securities, fair value	$ 9.8