Securities Available for Sale and Securities Held to Maturity (Available For Sale Securities with Unrealized Losses, Aggregated by Category and Period of Continuous Unrealized Loss) (Details) $ in Millions	Dec. 31, 2015 USD ($) security	Dec. 31, 2014 USD ($) security
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	$ 38,165	$ 11,028
Securities available for sale Less than 12 months Unrealized Losses	341	25
Securities available for sale 12 months or longer Fair Value	6,644	6,807
Securities available for sale 12 months or longer Unrealized Losses	91	58
Total Securities available for sale with unrealized losses Total Fair Value	44,809	17,835
Total Securities available for sale with unrealized losses Total Unrealized Losses	$ 432	$ 83
Number of available for sale securities in unrealized loss positions | security	409	173
U.S. agency mortgage-backed securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	$ 8,541	$ 2,247
Securities available for sale Less than 12 months Unrealized Losses	47	5
Securities available for sale 12 months or longer Fair Value	813	1,767
Securities available for sale 12 months or longer Unrealized Losses	1	7
Total Securities available for sale with unrealized losses Total Fair Value	9,354	4,014
Total Securities available for sale with unrealized losses Total Unrealized Losses	48	12
Asset-backed securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	17,127	5,754
Securities available for sale Less than 12 months Unrealized Losses	240	15
Securities available for sale 12 months or longer Fair Value	2,743	792
Securities available for sale 12 months or longer Unrealized Losses	66	3
Total Securities available for sale with unrealized losses Total Fair Value	19,870	6,546
Total Securities available for sale with unrealized losses Total Unrealized Losses	306	18
Corporate debt securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	5,433	1,781
Securities available for sale Less than 12 months Unrealized Losses	25	4
Securities available for sale 12 months or longer Fair Value	942	552
Securities available for sale 12 months or longer Unrealized Losses	6	4
Total Securities available for sale with unrealized losses Total Fair Value	6,375	2,333
Total Securities available for sale with unrealized losses Total Unrealized Losses	31	8
U.S. Treasury securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	5,010	1,246
Securities available for sale Less than 12 months Unrealized Losses	17	1
Total Securities available for sale with unrealized losses Total Fair Value	5,010	1,246
Total Securities available for sale with unrealized losses Total Unrealized Losses	17	1
U.S. agency notes [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	1,281
Securities available for sale Less than 12 months Unrealized Losses	10
Securities available for sale 12 months or longer Fair Value	1,547	3,696
Securities available for sale 12 months or longer Unrealized Losses	17	44
Total Securities available for sale with unrealized losses Total Fair Value	2,828	3,696
Total Securities available for sale with unrealized losses Total Unrealized Losses	27	$ 44
Certificates of deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	773
Securities available for sale Less than 12 months Unrealized Losses	2
Securities available for sale 12 months or longer Fair Value	599
Securities available for sale 12 months or longer Unrealized Losses	1
Total Securities available for sale with unrealized losses Total Fair Value	1,372
Total Securities available for sale with unrealized losses Total Unrealized Losses	$ 3