Investment Securities - Narrative (Details) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Pledged securities not separately reported	$ 52,000			$ 52,000		$ 46,000
Net impairment losses	$ 1		$ 1		$ 2
MBS and U.S. Treasury Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities sold		$ 2,700
Loss on sale		$ 40
U.S. Non-Agency Commercial Mortgage Backed Securities [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Gross pre-tax unrealized losses				$ 434
Number of securities in loss position | security				1,151