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	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning balance	$ 991	$ 861	$ 804
Included in earnings	(339)	(73)	(107)
Purchases	428	158	234
Settlements	(93)	(19)	(86)
Transfers into Level 3	36	64	16
Ending balance	1,023	991	861
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	(338)	(57)	(191)
Residential Mortgage
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning balance	179	137	143
Included in earnings	(1)	(3)	1
Purchases	0	0	0
Settlements	(31)	(19)	(23)
Transfers into Level 3	36	64	16
Ending balance	183	179	137
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	(1)	(3)	1
Mortgage Servicing Rights
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning balance	938	858	744
Included in earnings	(376)	(83)	(122)
Purchases	431	163	236
Settlements	0	0	0
Transfers into Level 3	0	0	0
Ending balance	993	938	858
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	(250)	(4)	(122)
Interest Rate Contract
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning balance	(1)	3	8
Included in earnings	145	72	94
Purchases	(3)	(5)	(2)
Settlements	(131)	(71)	(97)
Transfers into Level 3	0	0	0
Ending balance	10	(1)	3
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	20	9	10
Equity Contract
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
Beginning balance	(125)	(137)	(91)
Included in earnings	(107)	(59)	(80)
Purchases	0	0	0
Settlements	69	71	34
Transfers into Level 3	0	0	0
Ending balance	(163)	(125)	(137)
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	$ (107)	$ (59)	$ (80)