Securities - U.S. GSE, Agency, and Treasury Securities (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Marketable Securities [Line Items]
Amortized Cost	$ 392,512	$ 378,398
Fair Value	391,470	380,336
Fannie Mae
Marketable Securities [Line Items]
Amortized Cost	145,370	130,725
Fair Value	144,395	131,418
Government National Mortgage Association
Marketable Securities [Line Items]
Amortized Cost	90,844	98,278
Fair Value	90,049	98,633
U.S. Treasury
Marketable Securities [Line Items]
Amortized Cost	55,590	68,481
Fair Value	56,174	68,801
Freddie Mac
Marketable Securities [Line Items]
Amortized Cost	46,282	28,288
Fair Value	$ 46,059	$ 28,556