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, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Value, Assets	$ 36,747	$ 25,635
Fair Value, Assets	209	155
Notional Value, Liabilities	46,687	26,756
Fair Value, Liabilities	229	123
Other Derivatives [Member]
Derivative [Line Items]
Notional Value, Assets	825	$ 1
Fair Value, Assets	$ 5
Derivative Asset Average Remaining Maturity Period	3 days	3 days
Notional Value, Liabilities	$ 2,448	$ 1,458
Fair Value, Liabilities	$ 69	$ 84
Derivative Liability Average Remaining Maturity Period	8 months 1 day	1 year 6 months
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Receive Fixed/Pay Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 9,398	$ 3,605
Derivative Asset Average Remaining Maturity Period	9 years 9 months 21 days	14 years 9 months 18 days
Notional Value, Liabilities	$ 631	$ 4,333
Derivative Liability Average Remaining Maturity Period	4 years 4 months 20 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	$ 153	$ 4,333
Derivative Asset Average Remaining Maturity Period	16 years 1 month 9 days	6 years 11 months 19 days
Notional Value, Liabilities	$ 5,462	$ 1,132
Derivative Liability Average Remaining Maturity Period	6 years 1 month 6 days	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	$ 10,697	$ 2,839
Fair Value, Assets	$ 44	$ 27
Derivative Asset Average Remaining Maturity Period	21 days	25 days
Notional Value, Liabilities	$ 7,226	$ 1,140
Fair Value, Liabilities	$ 28	$ 5
Derivative Liability Average Remaining Maturity Period	21 days	18 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Futures and Forwards [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,742	$ 994
Fair Value, Assets	$ 8	$ 3
Derivative Asset Average Remaining Maturity Period	10 days	21 days
Notional Value, Liabilities	$ 18,490	$ 13,968
Fair Value, Liabilities	$ 63	$ 30
Derivative Liability Average Remaining Maturity Period	6 months 10 days	8 months 19 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 8,620	$ 5,080
Fair Value, Assets	$ 106	$ 88
Derivative Asset Average Remaining Maturity Period	3 years 8 months 12 days	10 years 9 months 7 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 2,077	$ 584
Fair Value, Assets	$ 43	$ 16
Derivative Asset Average Remaining Maturity Period	5 months 12 days	1 month 2 days
Notional Value, Liabilities	$ 2,819	$ 3
Fair Value, Liabilities	$ 54
Derivative Liability Average Remaining Maturity Period	1 year 10 months 9 days	1 month 2 days
Other Economic Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 264	$ 549
Fair Value, Assets	$ 1	$ 7
Derivative Asset Average Remaining Maturity Period	25 days	10 days
Notional Value, Liabilities	$ 654	$ 75
Fair Value, Liabilities	$ 6	$ 1
Derivative Liability Average Remaining Maturity Period	18 days	18 days
Other Economic Hedges [Member] | Equity Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 139	$ 19
Fair Value, Assets	$ 2	$ 1
Derivative Asset Average Remaining Maturity Period	10 months 6 days	9 months 25 days
Notional Value, Liabilities		$ 104
Fair Value, Liabilities		$ 2
Derivative Liability Average Remaining Maturity Period		5 months 12 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 6 months 7 days
Cash Flow Hedges [Member] | Interest Rate Contracts [Member] | Pay Fixed/Receive Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,532	$ 7,422
Fair Value, Assets		$ 8
Derivative Asset Average Remaining Maturity Period	6 years 6 months 21 days	3 years 1 month 9 days
Notional Value, Liabilities	$ 8,520	$ 4,320
Fair Value, Liabilities	$ 5
Derivative Liability Average Remaining Maturity Period	2 years 3 months 3 days	1 year 9 months 7 days
Net Investment Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative [Line Items]
Notional Value, Assets		$ 209
Fair Value, Assets		$ 5
Derivative Asset Average Remaining Maturity Period		18 days
Notional Value, Liabilities	$ 437	$ 223
Fair Value, Liabilities	$ 4	$ 1
Derivative Liability Average Remaining Maturity Period	18 days	18 days