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		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
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	$ 385	$ 465	$ 405	$ 478
Net Gains (Losses) Included in Net Income
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	$ (1)	$ 5	$ (2)	$ 12
Purchases
Sales
Principal Payments	$ (21)	$ (23)	$ (40)	$ (43)
Issuances
Settlements
End of Period Balance	$ 363	$ 447	$ 363	$ 447
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period	(1)	5	(2)	12
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	$ 273	297	$ 280	297
Net Gains (Losses) Included in Net Income		(3)		(4)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		$ 1		$ 8
Purchases
Sales
Principal Payments	$ (11)	$ (9)	$ (18)	$ (15)
Issuances
Settlements
End of Period Balance	$ 262	$ 286	$ 262	$ 286
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period				7
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	$ 61	$ 65	$ 62	63
Net Gains (Losses) Included in Net Income	1	$ 1	$ 2	2
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	$ (2)			1
Purchases		$ 2		$ 4
Sales
Principal Payments	$ (2)	$ (2)	$ (6)	$ (4)
Issuances
Settlements
End of Period Balance	$ 58	$ 66	$ 58	$ 66
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period	(2)			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
Net Gains (Losses) Included in Net Income
Net Gains (Losses) Included in Other Comprehensive Income (Loss)
Purchases
Sales
Principal Payments
Issuances
Settlements
End of Period Balance	$ 9	$ 9	$ 9	$ 9
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 2,250	$ 2,618	$ 2,338	$ 2,680
Net Gains (Losses) Included in Net Income	$ 58	$ (137)	$ (181)	$ (284)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)
Purchases	$ 9	$ 1	$ 15	$ 2
Sales		$ (141)		$ (141)
Principal Payments
Issuances	$ 164	$ 71	$ 309	$ 155
Settlements
End of Period Balance	$ 2,481	$ 2,412	$ 2,481	$ 2,412
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period	58	(137)	(181)	(284)
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	729	482	574	445
Net Gains (Losses) Included in Net Income	$ (72)	$ 288	$ 299	$ 473
Net Gains (Losses) Included in Other Comprehensive Income (Loss)
Purchases				$ 1
Sales	$ (4)		$ (5)
Principal Payments
Issuances
Settlements	$ (175)	$ (212)	$ (390)	$ (361)
End of Period Balance	478	558	478	558
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period	(131)	123	(6)	161
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	728	836	756	847
Net Gains (Losses) Included in Net Income	1	(2)	2	(2)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	$ (3)	6	$ (2)	21
Purchases		$ 2		$ 4
Sales
Principal Payments	$ (34)	$ (34)	$ (64)	$ (62)
Issuances
Settlements
End of Period Balance	$ 692	$ 808	$ 692	$ 808
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Still Held at End of Period	$ (3)	$ 5	$ (2)	$ 20