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, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Value, Assets	$ 36,566	$ 27,953
Fair Value, Assets	$ 199	155
Derivative Asset Average Remaining Maturity Period	0 years
Notional Value, Liabilities	$ 39,889	31,679
Fair Value, Liabilities	172	125
Other Derivatives [Member]
Derivative [Line Items]
Notional Value, Assets	210	$ 1
Fair Value, Assets	$ 3
Derivative Asset Average Remaining Maturity Period	7 days	3 days
Notional Value, Liabilities	$ 1,749	$ 1,458
Fair Value, Liabilities	$ 77	$ 84
Derivative Liability Average Remaining Maturity Period	1 year 1 month 6 days	1 year 6 months
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Receive Fixed/Pay Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 7,101	$ 3,605
Derivative Asset Average Remaining Maturity Period	10 years 7 months 9 days	14 years 9 months 18 days
Notional Value, Liabilities	$ 2,226	$ 4,333
Derivative Liability Average Remaining Maturity Period	11 years 3 months 7 days	6 years 11 months 19 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Pay Fixed/Receive Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,801	$ 4,333
Derivative Asset Average Remaining Maturity Period	12 years 2 months 19 days	6 years 11 months 19 days
Notional Value, Liabilities	$ 4,844	$ 1,132
Derivative Liability Average Remaining Maturity Period	6 years 3 months	7 years 7 months 20 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Futures and Forwards [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 5,062	$ 2,839
Fair Value, Assets	$ 34	$ 27
Derivative Asset Average Remaining Maturity Period	3 months 18 days	25 days
Notional Value, Liabilities	$ 2,357	$ 1,140
Fair Value, Liabilities	$ 5	$ 5
Derivative Liability Average Remaining Maturity Period	14 days	18 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Futures and Forwards [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 2,936	$ 994
Fair Value, Assets	$ 21	$ 3
Derivative Asset Average Remaining Maturity Period	14 days	21 days
Notional Value, Liabilities	$ 15,268	$ 13,968
Fair Value, Liabilities	$ 62	$ 30
Derivative Liability Average Remaining Maturity Period	5 months 19 days	8 months 19 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 9,945	$ 5,080
Fair Value, Assets	$ 97	$ 88
Derivative Asset Average Remaining Maturity Period	3 years 3 months 3 days	10 years 9 months 7 days
Notional Value, Liabilities	$ 300
Derivative Liability Average Remaining Maturity Period	5 years
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,105	$ 584
Fair Value, Assets	$ 30	$ 16
Derivative Asset Average Remaining Maturity Period	1 month 2 days	1 month 2 days
Notional Value, Liabilities	$ 1,006	$ 3
Fair Value, Liabilities	$ 20
Derivative Liability Average Remaining Maturity Period	1 year 11 months 23 days	1 month 2 days
Other Economic Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 324	$ 549
Fair Value, Assets	$ 1	$ 7
Derivative Asset Average Remaining Maturity Period	14 days	10 days
Notional Value, Liabilities	$ 362	$ 75
Fair Value, Liabilities	$ 2	$ 1
Derivative Liability Average Remaining Maturity Period	14 days	18 days
Other Economic Hedges [Member] | Equity Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 66	$ 19
Fair Value, Assets		$ 1
Derivative Asset Average Remaining Maturity Period	2 months 15 days	9 months 25 days
Notional Value, Liabilities	$ 70	$ 104
Fair Value, Liabilities		$ 2
Derivative Liability Average Remaining Maturity Period	3 months 18 days	5 months 12 days
Other Economic Hedges [Member] | Credit Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 2,377	$ 2,318
Fair Value, Assets	$ 1
Derivative Asset Average Remaining Maturity Period	3 years 3 months	3 years 6 months
Notional Value, Liabilities	$ 5,257	$ 4,923
Fair Value, Liabilities	$ 3	$ 2
Derivative Liability Average Remaining Maturity Period	4 years	4 years 14 days
Fair Value Hedges [Member] | Interest Rate Contracts [Member] | Receive Fixed/Pay Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,300
Derivative Asset Average Remaining Maturity Period	4 years 9 months 7 days
Derivative Liability Average Remaining Maturity Period	0 years
Cash Flow Hedges [Member] | Interest Rate Contracts [Member] | Pay Fixed/Receive Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 3,892	$ 7,422
Fair Value, Assets	$ 10	$ 8
Derivative Asset Average Remaining Maturity Period	5 years 5 months 23 days	3 years 1 month 9 days
Notional Value, Liabilities	$ 6,450	$ 4,320
Fair Value, Liabilities	$ 3
Derivative Liability Average Remaining Maturity Period	1 year 3 months 14 days	1 year 9 months 7 days
Net Investment Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 447	$ 209
Fair Value, Assets	$ 2	$ 5
Derivative Asset Average Remaining Maturity Period	18 days	18 days
Notional Value, Liabilities		$ 223
Fair Value, Liabilities		$ 1
Derivative Liability Average Remaining Maturity Period	0 years	18 days