Fair Values of Assets and Liabilities - Changes in Fair Value for All Assets and Liabilities Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Non-Agency Prime [Member] | Mortgage-Backed Securities Residential [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 318	$ 405
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	(3)	(1)
Sales	0	0
Principal Payments	(18)	(19)
Settlements	0	0
End of Period Balance	297	385
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period	(3)	(1)
Non-Agency Non-Prime [Member] | Mortgage-Backed Securities Residential [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	240	280
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	(5)
Sales	0	0
Principal Payments	(8)	(7)
Settlements	0	0
End of Period Balance	227	273
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period	(5)
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	9	9
Sales	0	0
Settlements	0	0
End of Period Balance	9	9
Asset-Backed Securities Other [Member] | Asset-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	2	62
Net Gains (Losses) Included in Net Income		1
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		2
Sales	0	0
Principal Payments		(4)
Settlements	0	0
End of Period Balance	2	61
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period		2
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	2,512	2,338
Net Gains (Losses) Included in Net Income	(398)	(239)
Purchases	9	6
Sales	0	0
Issuances	99	145
Settlements	0	0
End of Period Balance	2,222	2,250
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period	(398)	(239)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	569	756
Net Gains (Losses) Included in Net Income		1
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	(8)	1
Sales	0	0
Principal Payments	(26)	(30)
Settlements	0	0
End of Period Balance	535	728
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period	(8)	1
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	498	574
Net Gains (Losses) Included in Net Income	502	371
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0
Purchases	0	0
Sales	(2)	(1)
Principal Payments	0	0
Issuances	0	0
Settlements	(147)	(215)
End of Period Balance	851	729
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period	$ 369	$ 243