Securities (Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Securities
AFS securities, debt securities due in one year or less, amortized cost	$ 265
AFS securities, debt securities due in one year or less, fair value	264
AFS securities, debt securities due after one year through five years, amortized cost	1,661
AFS securities, debt securities due after one year through five years, fair value	1,667
AFS securities, debt securities due after five years through ten years, amortized cost	971
AFS securities, debt securities due after five years through ten years, fair value	992
AFS securities, debt securities due after ten years, amortized cost	22,452
AFS securities, debt securities due after ten years, fair value	22,374
Total AFS debt securities, amortized cost	25,349
Total AFS debt securities, fair value	25,297
HTM securities, debt securities due in one year or less, amortized cost	1
HTM securities, debt securities due in one year or less, fair value	1
HTM securities, debt securities due after one year through five years, amortized cost	2,097
HTM securities, debt securities due after one year through five years, fair value	2,101
HTM securities, debt securities due after five years through ten years, amortized cost	4,062
HTM securities, debt securities due after five years through ten years, fair value	3,998
HTM securities, debt securities due after ten years, amortized cost	12,370
HTM securities, debt securities due after ten years, fair value	12,419
Total HTM securities, amortized cost	18,530	$ 20,240
Total HTM securities, fair value	$ 18,519	$ 20,313