Investment Securities - Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) security	Jun. 30, 2016 USD ($)	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		$ 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			$ 527
Number of securities in loss position | security			1,309