Derivative Instruments - Asset and Liability Management Derivative Positions of Company (Detail) - Asset and Liability Management Positions [Member] - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Value, Assets	$ 47,709	$ 25,635
Fair Value, Assets	150	155
Notional Value, Liabilities	65,657	26,756
Fair Value, Liabilities	213	123
Other Derivatives [Member]
Derivative [Line Items]
Notional Value, Assets	675	$ 1
Fair Value, Assets	$ 6
Derivative Asset Average Remaining Maturity Period	3 days	3 days
Notional Value, Liabilities	$ 2,394	$ 1,458
Fair Value, Liabilities	$ 58	$ 84
Derivative Liability Average Remaining Maturity Period	6 months 14 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	$ 5,762	$ 3,605
Derivative Asset Average Remaining Maturity Period	8 years 6 months 14 days	14 years 9 months 18 days
Notional Value, Liabilities	$ 5,202	$ 4,333
Derivative Liability Average Remaining Maturity Period	10 years 4 months 6 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	$ 964	$ 4,333
Derivative Asset Average Remaining Maturity Period	6 years 11 months 8 days	6 years 11 months 19 days
Notional Value, Liabilities	$ 5,220	$ 1,132
Derivative Liability Average Remaining Maturity Period	5 years 11 months 4 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	$ 7,373	$ 2,839
Fair Value, Assets	$ 21	$ 27
Derivative Asset Average Remaining Maturity Period	21 days	25 days
Notional Value, Liabilities	$ 14,083	$ 1,140
Fair Value, Liabilities	$ 52	$ 5
Derivative Liability Average Remaining Maturity Period	1 month 2 days	18 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Futures and Forwards [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 5,169	$ 994
Fair Value, Assets	$ 13	$ 3
Derivative Asset Average Remaining Maturity Period	10 days	21 days
Notional Value, Liabilities	$ 22,883	$ 13,968
Fair Value, Liabilities	$ 20	$ 30
Derivative Liability Average Remaining Maturity Period	9 months 14 days	8 months 19 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 9,300	$ 5,080
Fair Value, Assets	$ 57	$ 88
Derivative Asset Average Remaining Maturity Period	2 years 2 months 8 days	10 years 9 months 7 days
Notional Value, Liabilities	$ 200
Derivative Liability Average Remaining Maturity Period	1 year 3 days
Other Economic Hedges [Member] | Interest Rate Contracts [Member] | Options [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 1,959	$ 584
Fair Value, Assets	$ 47	$ 16
Derivative Asset Average Remaining Maturity Period	1 month 6 days	1 month 2 days
Notional Value, Liabilities	$ 4,010	$ 3
Fair Value, Liabilities	$ 76
Derivative Liability Average Remaining Maturity Period	1 year 9 months 21 days	1 month 2 days
Other Economic Hedges [Member] | Foreign Exchange Forward Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 242	$ 549
Fair Value, Assets	$ 1	$ 7
Derivative Asset Average Remaining Maturity Period	14 days	10 days
Notional Value, Liabilities	$ 437	$ 75
Fair Value, Liabilities	$ 1	$ 1
Derivative Liability Average Remaining Maturity Period	18 days	18 days
Other Economic Hedges [Member] | Equity Contracts [Member]
Derivative [Line Items]
Notional Value, Assets	$ 18	$ 19
Fair Value, Assets	$ 1	$ 1
Derivative Asset Average Remaining Maturity Period	7 months 20 days	9 months 25 days
Notional Value, Liabilities	$ 126	$ 104
Fair Value, Liabilities	$ 2	$ 2
Derivative Liability Average Remaining Maturity Period	7 months 20 days	5 months 12 days
Fair Value Hedges [Member] | Interest Rate Contracts [Member] | Receive Fixed/Pay Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 15,300
Derivative Asset Average Remaining Maturity Period	4 years 10 months 28 days
Notional Value, Liabilities	$ 1,900
Derivative Liability Average Remaining Maturity Period	3 years 11 months 19 days
Cash Flow Hedges [Member] | Interest Rate Contracts [Member] | Pay Fixed/Receive Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 500	$ 7,422
Fair Value, Assets		$ 8
Derivative Asset Average Remaining Maturity Period	2 months 15 days	3 years 1 month 9 days
Notional Value, Liabilities	$ 9,202	$ 4,320
Fair Value, Liabilities	$ 4
Derivative Liability Average Remaining Maturity Period	2 years 10 months 24 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	$ 4	$ 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		18 days