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		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 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	$ 280	$ 363	$ 318	$ 405
Net Gains (Losses) Included in Net Income			(1)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)				(2)
Sales	0	0	0	0
Principal Payments	(21)	(25)	(58)	(65)
Settlements	0	0	0	0
End of Period Balance	259	338	259	338
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period				(2)
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	216	262	240	280
Net Gains (Losses) Included in Net Income		(1)	(1)	(1)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	(1)	1	(4)	1
Sales	0	0	0	0
Principal Payments	(9)	(11)	(29)	(29)
Settlements	0	0	0	0
End of Period Balance	206	251	206	251
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(1)	1	(4)	1
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	9	9	9	9
Sales	0	0	0	0
Settlements	0	0	0	0
End of Period Balance	9	9	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	58	2	62
Net Gains (Losses) Included in Net Income		2		4
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		(1)		(1)
Sales	0	0	0	0
Principal Payments		(3)		(9)
Settlements	0	0	0	0
End of Period Balance	2	56	2	56
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period		(1)		(1)
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	2,056	2,481	2,512	2,338
Net Gains (Losses) Included in Net Income	(85)	(273)	(785)	(454)
Purchases	18	7	32	22
Sales	0	0	0	0
Issuances	142	182	372	491
Settlements	0	0	0	0
End of Period Balance	2,131	2,397	2,131	2,397
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(85)	(273)	(785)	(454)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	507	692	569	756
Net Gains (Losses) Included in Net Income		1	(2)	3
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	(1)		(4)	(2)
Sales	0	0	0	0
Principal Payments	(30)	(39)	(87)	(103)
Settlements	0	0	0	0
End of Period Balance	476	654	476	654
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(1)		(4)	(2)
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	1,080	478	498	574
Net Gains (Losses) Included in Net Income	84	426	1,047	725
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases			1
Sales	(2)	(2)	(5)	(7)
Principal Payments	0	0	0	0
Issuances	0	0	0	0
Settlements	(231)	(219)	(610)	(609)
End of Period Balance	931	683	931	683
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	$ 16	$ 231	$ 494	$ 203