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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	$ 2,780	$ 2,642	$ 2,645	2,591
Net Gains (Losses) Included in Net Income	(35)	(146)	(2)	(219)
Purchases	2	4	4	6
Issuances	97	82	197	204
Settlements	0	0	0	0
End of Period Balance	2,844	2,582	2,844	2,582
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	(35)	(146)	(2)	(219)
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	(132)	165	107	171
Net Gains (Losses) Included in Net Income	(94)	215	(345)	261
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0	0	0
Purchases			1	1
Sales	(16)	(2)	(22)	(5)
Principal Payments	0	0	0	0
Issuances	0	0	0	0
Settlements	(14)	(138)	3	(188)
End of Period Balance	(256)	240	(256)	240
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	$ (110)	$ 117	$ (297)	$ 74