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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
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	$ 242	$ 318
Net Gains (Losses) Included in Net Income				(1)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)			(2)
Sales			(234)
Principal Payments		(21)	(6)	(58)
Settlements		0	0	0
End of Period Balance		259		259
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	195	240
Net Gains (Losses) Included in Net Income				(1)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		(1)	(17)	(4)
Sales			(175)
Principal Payments		(9)	(3)	(29)
Settlements		0	0	0
End of Period Balance		206		206
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period		(1)		(4)
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance		9	9	9
Net Gains (Losses) Included in Other Comprehensive Income (Loss)			2
Sales			(11)
Principal Payments			0
Settlements		0	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	2	2
Sales			(2)
Principal Payments			0
Settlements		0	0	0
End of Period Balance		2		2
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 2,582	2,056	2,591	2,512
Net Gains (Losses) Included in Net Income	(103)	(85)	(322)	(785)
Purchases	4	18	10	32
Issuances	115	142	319	372
Settlements	0	0	0	0
End of Period Balance	2,598	2,131	2,598	2,131
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(103)	(85)	(322)	(785)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance		507	448	569
Net Gains (Losses) Included in Net Income				(2)
Net Gains (Losses) Included in Other Comprehensive Income (Loss)		(1)	(17)	(4)
Sales			(422)
Principal Payments		(30)	(9)	(87)
Settlements		0	0	0
End of Period Balance		476		476
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period		(1)		(4)
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	240	1,080	171	498
Net Gains (Losses) Included in Net Income	111	84	372	1,047
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases			1	1
Sales	(3)	(2)	(8)	(5)
Principal Payments	0	0	0	0
Issuances	0	0	0	0
Settlements	(95)	(231)	(283)	(610)
End of Period Balance	253	931	253	931
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	$ 41	$ 16	$ 87	$ 494