Securities - Narrative (Details) $ in Billions	Dec. 31, 2017 USD ($) security
FNMA investments
Schedule of Available-for-sale Securities [Line Items]
Securities, amortized cost	$ 14.7
Securities, fair value	14.4
FHLMC investments
Schedule of Available-for-sale Securities [Line Items]
Securities, amortized cost	10.2
Securities, fair value	$ 10.0
Non-agency MBS
Schedule of Available-for-sale Securities [Line Items]
Number of securities with other than temporary credit impairment | security	0