Fair Value Measurements (Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning balance	$ 1,023	$ 991
Included in earnings	(293)	(91)
Purchases	69	286
Settlements	(46)	(11)
Transfers into Level 3	7	15
Ending balance	760	1,190
The amount of total gains or losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held	(289)	(89)
Residential Mortgage
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning balance	183	179
Included in earnings	4	0
Purchases	0	0
Settlements	(9)	(4)
Transfers into Level 3	7	15
Ending balance	185	190
The amount of total gains or losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held	4	0
Mortgage Servicing Rights
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning balance	993	938
Included in earnings	(378)	(84)
Purchases	70	287
Settlements	0	0
Transfers into Level 3	0	0
Ending balance	685	1,141
The amount of total gains or losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held	(341)	(69)
Interest Rate Contract
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning balance	10	(1)
Included in earnings	103	24
Purchases	(1)	(1)
Settlements	(51)	(20)
Transfers into Level 3	0	0
Ending balance	61	2
The amount of total gains or losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held	70	11
Equity Contract
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning balance	(163)	(125)
Included in earnings	(22)	(31)
Purchases	0	0
Settlements	14	13
Transfers into Level 3	0	0
Ending balance	(171)	(143)
The amount of total gains or losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held	$ (22)	$ (31)