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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Non-Agency Prime [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance				$ 242
Net Gains (Losses) Included in Other Comprehensive Income (Loss)				(2)
Sales				(234)
Principal Payments				(6)
Settlements				0
Non-Agency Non-Prime [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance				195
Net Gains (Losses) Included in Other Comprehensive Income (Loss)				(17)
Sales				(175)
Principal Payments				(3)
Settlements				0
Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance				9
Net Gains (Losses) Included in Other Comprehensive Income (Loss)				2
Sales				(11)
Settlements				0
Other Asset-Backed Securities [Member] | Asset-Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance				2
Sales				(2)
Settlements				0
Mortgage Servicing Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	$ 2,844	$ 2,582	$ 2,645	2,591
Net Gains (Losses) Included in Net Income	(23)	(103)	(25)	(322)
Purchases	2	4	6	10
Sales	(15)		(15)
Issuances	109	115	306	319
Settlements	0	0	0	0
End of Period Balance	2,917	2,598	2,917	2,598
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(23)	(103)	(25)	(322)
Available-for-Sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance				448
Net Gains (Losses) Included in Other Comprehensive Income (Loss)				(17)
Sales				(422)
Principal Payments				(9)
Settlements				0
Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning of Period Balance	(256)	240	107	171
Net Gains (Losses) Included in Net Income	(81)	111	(426)	372
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases	2		3	1
Sales	(13)	(3)	(35)	(8)
Principal Payments	0	0	0	0
Issuances	0	0	0	0
Settlements	(10)	(95)	(7)	(283)
End of Period Balance	(358)	253	(358)	253
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	$ (81)	$ 41	$ (402)	$ 87