Summary of Significant Accounting Policies - Income Statement (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Servicing fees	$ 5,153	$ 5,108	$ 4,799
Total revenue	10,360	10,274	9,864
Income before income taxes	2,298	2,437	2,666
Income tax expense	318	415	616
Net income	$ 1,980	$ 2,022	$ 2,050
Basic (in dollars per share)	$ 4.53	$ 4.62	$ 4.52
Diluted (in dollars per share)	$ 4.47	$ 4.53	$ 4.43
Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Servicing fees		$ 5,129	$ 4,819
Total revenue		10,295	9,884
Income before income taxes		2,458	2,686
Income tax expense		421	550
Net income		$ 2,037	$ 2,136
Basic (in dollars per share)		$ 4.65	$ 4.71
Diluted (in dollars per share)		$ 4.57	$ 4.62
Correction
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Servicing fees		$ (21)	$ (20)
Total revenue		(21)	(20)
Income before income taxes		(21)	(20)
Income tax expense		(6)	66
Net income		$ (15)	$ (86)
Basic (in dollars per share)		$ (0.03)	$ (0.19)
Diluted (in dollars per share)		$ (0.04)	$ (0.19)