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
	Mar. 31, 2018	Mar. 31, 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	$ 2,645	2,591
Net Gains (Losses) Included in Net Income	33	(73)
Purchases	2	2
Issuances	100	122
Settlements	0	0
End of Period Balance	2,780	2,642
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	33	(73)
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	107	171
Net Gains (Losses) Included in Net Income	(251)	46
Net Gains (Losses) Included in Other Comprehensive Income (Loss)	0	0
Purchases	1	1
Sales	(6)	(3)
Principal Payments	0	0
Issuances	17
Settlements		(50)
End of Period Balance	(132)	165
Net Change in Unrealized Gains (Losses) Relating to Assets and Liabilities Held at End of Period	$ (212)	$ (7)