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, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Value, Assets	$ 41,815	$ 36,579
Fair Value, Assets	264	163
Notional Value, Liabilities	22,221	21,030
Fair Value, Liabilities	305	172
Other Derivatives [Member]
Derivative [Line Items]
Notional Value, Assets	300
Fair Value, Assets	$ 5
Derivative Asset Average Remaining Maturity Period	7 days
Notional Value, Liabilities	$ 1,514	1,164
Fair Value, Liabilities	$ 127	$ 125
Derivative Liability Average Remaining Maturity Period	1 year 9 months 22 days	2 years 6 months
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Receive Fixed/Pay Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 2,750	$ 2,019
Fair Value, Assets		$ 5
Derivative Asset Average Remaining Maturity Period	15 years 11 days	16 years 5 months 27 days
Notional Value, Liabilities	$ 4,420	$ 5,469
Fair Value, Liabilities	$ 114
Derivative Liability Average Remaining Maturity Period	7 years 11 months 15 days	8 years 5 months 5 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Pay Fixed/Receive Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 4,333	$ 4,844
Fair Value, Assets		$ 21
Derivative Asset Average Remaining Maturity Period	7 years 8 months 19 days	7 years 8 months 9 days
Notional Value, Liabilities	$ 88	$ 46
Fair Value, Liabilities		$ 1
Derivative Liability Average Remaining Maturity Period	5 years 2 months 23 days	6 years 8 months 12 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Futures and Forwards [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,895	$ 1,632
Fair Value, Assets	$ 8	$ 7
Derivative Asset Average Remaining Maturity Period	1 month 6 days	1 month 6 days
Notional Value, Liabilities	$ 1,172	$ 1,326
Fair Value, Liabilities	$ 2	$ 2
Derivative Liability Average Remaining Maturity Period	1 month 2 days	15 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Futures and Forwards [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 19,126	$ 15,291
Fair Value, Assets	$ 19	$ 10
Derivative Asset Average Remaining Maturity Period	1 year 1 month 20 days	10 months 21 days
Notional Value, Liabilities	$ 5,193	$ 4,511
Fair Value, Liabilities	$ 17	$ 10
Derivative Liability Average Remaining Maturity Period	7 days	11 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 6,235	$ 4,985
Fair Value, Assets	$ 77	$ 65
Derivative Asset Average Remaining Maturity Period	7 years 7 months 17 days	7 years 6 months 25 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,123	$ 1,285
Fair Value, Assets	$ 26	$ 21
Derivative Asset Average Remaining Maturity Period	1 month 2 days	1 month 6 days
Notional Value, Liabilities	$ 7	$ 5
Derivative Liability Average Remaining Maturity Period	1 month 13 days	18 days
Other Economic Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 132	$ 147
Fair Value, Assets	$ 1	$ 1
Derivative Asset Average Remaining Maturity Period	18 days	7 days
Notional Value, Liabilities	$ 540	$ 669
Fair Value, Liabilities	$ 3	$ 8
Derivative Liability Average Remaining Maturity Period	15 days	15 days
Other Economic Hedges [Member] | Equity Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 25	$ 45
Fair Value, Assets	$ 1
Derivative Asset Average Remaining Maturity Period	6 months 25 days	1 year 1 month 6 days
Notional Value, Liabilities	$ 108	$ 88
Fair Value, Liabilities	$ 4	$ 1
Derivative Liability Average Remaining Maturity Period	6 months	6 months 29 days
Other Economic Hedges [Member] | Credit Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,812	$ 1,559
Derivative Asset Average Remaining Maturity Period	3 years 7 days	3 years 4 months 28 days
Notional Value, Liabilities	$ 3,880	$ 3,779
Fair Value, Liabilities	$ 1	$ 1
Derivative Liability Average Remaining Maturity Period	3 years 1 month 24 days	3 years 1 month 27 days
Fair Value Hedges [Member] | Interest Rate Contracts [Member] | Receive Fixed/Pay Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets		$ 1,000
Fair Value, Assets		$ 28
Derivative Asset Average Remaining Maturity Period		6 years 8 months 12 days
Notional Value, Liabilities	$ 4,600	$ 3,600
Fair Value, Liabilities	$ 37	$ 16
Derivative Liability Average Remaining Maturity Period	2 years 5 months 1 day	1 year 6 months 18 days
Cash Flow Hedges [Member] | Interest Rate Contracts [Member] | Pay Fixed/Receive Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 3,892	$ 3,772
Fair Value, Assets	$ 126	$ 5
Derivative Asset Average Remaining Maturity Period	6 years 5 months 23 days	6 years 8 months 23 days
Notional Value, Liabilities	$ 500
Derivative Liability Average Remaining Maturity Period	7 months 17 days
Net Investment Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 192
Fair Value, Assets	$ 1
Derivative Asset Average Remaining Maturity Period	18 days
Notional Value, Liabilities	$ 199	$ 373
Fair Value, Liabilities		$ 8
Derivative Liability Average Remaining Maturity Period	18 days	18 days