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) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015 security	Dec. 31, 2014 security
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	$ 11,393	$ 11,028
Securities available for sale Less than 12 months Unrealized Losses	25	25
Securities available for sale 12 months or longer Fair Value	4,794	6,807
Securities available for sale 12 months or longer Unrealized Losses	21	58
Total Securities available for sale with unrealized losses Total Fair Value	16,187	17,835
Total Securities available for sale with unrealized losses Total Unrealized Losses	46	83
Number of available for sale securities in unrealized loss positions	174	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	3,247	2,247
Securities available for sale Less than 12 months Unrealized Losses	3	5
Securities available for sale 12 months or longer Fair Value	1,135	1,767
Securities available for sale 12 months or longer Unrealized Losses	3	7
Total Securities available for sale with unrealized losses Total Fair Value	4,382	4,014
Total Securities available for sale with unrealized losses Total Unrealized Losses	6	12
Asset-backed securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	5,694	5,754
Securities available for sale Less than 12 months Unrealized Losses	19	15
Securities available for sale 12 months or longer Fair Value	750	792
Securities available for sale 12 months or longer Unrealized Losses	5	3
Total Securities available for sale with unrealized losses Total Fair Value	6,444	6,546
Total Securities available for sale with unrealized losses Total Unrealized Losses	24	18
Corporate debt securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	1,079	1,781
Securities available for sale Less than 12 months Unrealized Losses	1	4
Securities available for sale 12 months or longer Fair Value	655	552
Securities available for sale 12 months or longer Unrealized Losses	2	4
Total Securities available for sale with unrealized losses Total Fair Value	1,734	2,333
Total Securities available for sale with unrealized losses Total Unrealized Losses	3	8
U.S. agency notes [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value	1,373
Securities available for sale Less than 12 months Unrealized Losses	2
Securities available for sale 12 months or longer Fair Value	2,254	3,696
Securities available for sale 12 months or longer Unrealized Losses	11	44
Total Securities available for sale with unrealized losses Total Fair Value	3,627	3,696
Total Securities available for sale with unrealized losses Total Unrealized Losses	13	44
Treasury securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Securities available for sale Less than 12 months Fair Value		1,246
Securities available for sale Less than 12 months Unrealized Losses		1
Total Securities available for sale with unrealized losses Total Fair Value		1,246
Total Securities available for sale with unrealized losses Total Unrealized Losses		$ 1