Investment Securities - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) security	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Pledged securities not separately reported	$ 48,000		$ 46,000
Net impairment losses	1	$ 1
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	$ 646
Number of securities in loss position | security	1,389