Accumulated Other Comprehensive Income (Accumulated Other Comprehensive Income Balances) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income
Beginning Balance	$ (152)	$ (163)
Other net changes	(75)	31
Ending Balance	(260)	(132)
Net unrealized gain (loss) on available for sale securities [Member]
Accumulated Other Comprehensive Income
Other net changes	(82)	33
Reclassification of Securities Transferred to Held to Maturity [Member]
Accumulated Other Comprehensive Income
Other net changes		142
Other Reclassifications in Other Revenue Available for Sale Securities [Member]
Accumulated Other Comprehensive Income
Other net changes		(1)
Reclassification of Securities Transferred from Available for Sale [Member]
Accumulated Other Comprehensive Income
Other net changes		(142)
Amortization of amounts previously recorded upon transfer to held to maturity from available for sale [Member]
Accumulated Other Comprehensive Income
Other net changes	7	1
Other [Member]
Accumulated Other Comprehensive Income
Other net changes		$ (2)
Adoption of New Accounting Standards [Member] | ASU 2018-02 [Member]
Accumulated Other Comprehensive Income
Other net changes	$ (33)