Derivative Instruments - Asset and Liability Management Derivative Positions of Company (Detail) - Asset and Liability Management Positions [Member] - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Value, Assets	$ 42,650	$ 36,579
Fair Value, Assets	158	163
Notional Value, Liabilities	19,358	21,030
Fair Value, Liabilities	151	172
Other Derivatives [Member]
Derivative [Line Items]
Notional Value, Assets	147
Fair Value, Assets	$ 1
Derivative Asset Average Remaining Maturity Period	1 month 6 days
Notional Value, Liabilities	$ 1,420	1,164
Fair Value, Liabilities	$ 120	$ 125
Derivative Liability Average Remaining Maturity Period	1 year 9 months 18 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	$ 2,822	$ 2,019
Fair Value, Assets		$ 5
Derivative Asset Average Remaining Maturity Period	12 years 9 months 3 days	16 years 5 months 27 days
Notional Value, Liabilities	$ 4,911	$ 5,469
Derivative Liability Average Remaining Maturity Period	9 years 26 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	$ 4,530	$ 4,844
Fair Value, Assets		$ 21
Derivative Asset Average Remaining Maturity Period	7 years 4 months 13 days	7 years 8 months 9 days
Notional Value, Liabilities	$ 430	$ 46
Fair Value, Liabilities		$ 1
Derivative Liability Average Remaining Maturity Period	11 years 3 months 19 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	$ 2,363	$ 1,632
Fair Value, Assets	$ 12	$ 7
Derivative Asset Average Remaining Maturity Period	1 month 2 days	1 month 6 days
Notional Value, Liabilities	$ 602	$ 1,326
Fair Value, Liabilities	$ 2	$ 2
Derivative Liability Average Remaining Maturity Period	1 month 6 days	15 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Futures and Forwards [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 13,904	$ 15,291
Fair Value, Assets	$ 9	$ 10
Derivative Asset Average Remaining Maturity Period	1 year 1 month 6 days	10 months 21 days
Notional Value, Liabilities	$ 5,718	$ 4,511
Fair Value, Liabilities	$ 23	$ 10
Derivative Liability Average Remaining Maturity Period	11 days	11 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 6,255	$ 4,985
Fair Value, Assets	$ 85	$ 65
Derivative Asset Average Remaining Maturity Period	8 years 2 months 5 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	$ 1,060	$ 1,285
Fair Value, Assets	$ 25	$ 21
Derivative Asset Average Remaining Maturity Period	1 month 2 days	1 month 6 days
Notional Value, Liabilities	$ 8	$ 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	$ 579	$ 147
Fair Value, Assets	$ 8	$ 1
Derivative Asset Average Remaining Maturity Period	18 days	7 days
Notional Value, Liabilities	$ 141	$ 669
Fair Value, Liabilities	$ 1	$ 8
Derivative Liability Average Remaining Maturity Period	18 days	15 days
Other Economic Hedges [Member] | Equity Contracts [Member]
Derivative [Line Items]
Notional Value, Assets		$ 45
Derivative Asset Average Remaining Maturity Period		1 year 1 month 6 days
Notional Value, Liabilities	$ 143	$ 88
Fair Value, Liabilities	$ 4	$ 1
Derivative Liability Average Remaining Maturity Period	10 months 3 days	6 months 29 days
Other Economic Hedges [Member] | Credit Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,820	$ 1,559
Derivative Asset Average Remaining Maturity Period	2 years 9 months 29 days	3 years 4 months 28 days
Notional Value, Liabilities	$ 4,265	$ 3,779
Fair Value, Liabilities	$ 1	$ 1
Derivative Liability Average Remaining Maturity Period	3 years 5 months 16 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		$ 3,600
Fair Value, Liabilities		$ 16
Derivative Liability Average Remaining Maturity Period		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	$ 8,872	$ 3,772
Fair Value, Assets	$ 14	$ 5
Derivative Asset Average Remaining Maturity Period	3 years 3 months 11 days	6 years 8 months 23 days
Notional Value, Liabilities	$ 1,720
Derivative Liability Average Remaining Maturity Period	2 years 4 months 6 days
Net Investment Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 298
Fair Value, Assets	$ 4
Derivative Asset Average Remaining Maturity Period	18 days
Notional Value, Liabilities		$ 373
Fair Value, Liabilities		$ 8
Derivative Liability Average Remaining Maturity Period		18 days