Derivative Instruments - Asset and Liability Management Derivative Positions of Company (Detail) - Asset and Liability Management Positions [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional Value, Assets	$ 77,241	$ 58,966
Fair Value, Assets	829	142
Notional Value, Liabilities	74,638	42,288
Fair Value, Liabilities	1,051	301
Other Derivatives [Member]
Derivative [Line Items]
Notional Value, Assets	1,031	$ 34
Fair Value, Assets	$ 29
Derivative Asset Average Remaining Maturity Period	3 days	3 days
Notional Value, Liabilities	$ 2,820	$ 1,823
Fair Value, Liabilities	$ 181	$ 165
Derivative Liability Average Remaining Maturity Period	1 year 5 months 4 days	2 years 5 months 12 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Receive Fixed/Pay Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 16,135	$ 4,408
Derivative Asset Average Remaining Maturity Period	8 years 7 months 17 days	5 years 11 months 26 days
Notional Value, Liabilities	$ 2,032	$ 5,316
Derivative Liability Average Remaining Maturity Period	7 years 1 month 2 days	13 years 14 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Pay Fixed/Receive Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,827	$ 1,259
Derivative Asset Average Remaining Maturity Period	13 years 5 months 23 days	5 years 8 months 1 day
Notional Value, Liabilities	$ 10,044	$ 4,497
Derivative Liability Average Remaining Maturity Period	6 years 10 days	6 years 10 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Futures and Forwards [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 21,970	$ 5,409
Fair Value, Assets	$ 484	$ 17
Derivative Asset Average Remaining Maturity Period	18 days	29 days
Notional Value, Liabilities	$ 15,091	$ 5,477
Fair Value, Liabilities	$ 339	$ 11
Derivative Liability Average Remaining Maturity Period	14 days	25 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Futures and Forwards [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 3,542	$ 16,333
Fair Value, Assets	$ 75	$ 13
Derivative Asset Average Remaining Maturity Period	10 days	9 months 21 days
Notional Value, Liabilities	$ 29,313	$ 8,113
Fair Value, Liabilities	$ 316	$ 25
Derivative Liability Average Remaining Maturity Period	7 months 6 days	10 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 3,830	$ 10,180
Fair Value, Assets	$ 87	$ 79
Derivative Asset Average Remaining Maturity Period	5 years 2 months 26 days	2 years 11 months 19 days
Notional Value, Liabilities	$ 200
Derivative Liability Average Remaining Maturity Period	6 months 3 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 4,201	$ 1,270
Fair Value, Assets	$ 140	$ 30
Derivative Asset Average Remaining Maturity Period	1 month 28 days	29 days
Notional Value, Liabilities	$ 5,873	$ 4,238
Fair Value, Liabilities	$ 161	$ 81
Derivative Liability Average Remaining Maturity Period	2 years 4 months 9 days	2 years 25 days
Other Economic Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 355	$ 113
Fair Value, Assets	$ 10	$ 1
Derivative Asset Average Remaining Maturity Period	10 days	18 days
Notional Value, Liabilities	$ 350	$ 467
Fair Value, Liabilities	$ 4	$ 6
Derivative Liability Average Remaining Maturity Period	18 days	14 days
Other Economic Hedges [Member] | Equity Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 92	$ 128
Fair Value, Assets	$ 4	$ 2
Derivative Asset Average Remaining Maturity Period	3 months 10 days	5 months 12 days
Notional Value, Liabilities	$ 19	$ 20
Fair Value, Liabilities	$ 4
Derivative Liability Average Remaining Maturity Period	1 month 20 days	1 year 21 days
Fair Value Hedges [Member] | Interest Rate Contracts [Member] | Receive Fixed/Pay Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 24,200	$ 18,300
Derivative Asset Average Remaining Maturity Period	3 years 7 months 9 days	3 years 10 months 20 days
Notional Value, Liabilities		$ 4,900
Derivative Liability Average Remaining Maturity Period		3 years 5 months 26 days
Cash Flow Hedges [Member] | Interest Rate Contracts [Member] | Pay Fixed/Receive Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets		$ 1,532
Derivative Asset Average Remaining Maturity Period		6 years 21 days
Notional Value, Liabilities	$ 8,682	$ 7,150
Fair Value, Liabilities	$ 45	$ 10
Derivative Liability Average Remaining Maturity Period	2 years 6 months 18 days	2 years 1 month 9 days
Net Investment Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 58
Derivative Asset Average Remaining Maturity Period	14 days
Notional Value, Liabilities	$ 214	$ 287
Fair Value, Liabilities	$ 1	$ 3
Derivative Liability Average Remaining Maturity Period	14 days	14 days