Derivative Instruments - Customer-Related Derivative Positions of Company (Detail) - Customer-Related Positions [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Value, Assets	$ 171,661	$ 156,822
Fair Value, Assets	1,866	2,319
Notional Value, Liabilities	165,812	140,485
Fair Value, Liabilities	1,947	2,195
Interest Rate Contracts [Member] | Receive Fixed/Pay Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	25,579	28,681
Fair Value, Assets	$ 492	$ 679
Derivative Asset Average Remaining Maturity Period	4 years 4 months 28 days	5 years 8 months 16 days
Notional Value, Liabilities	$ 75,797	$ 59,990
Fair Value, Liabilities	$ 630	$ 840
Derivative Liability Average Remaining Maturity Period	4 years 5 months 12 days	4 years 3 months 8 days
Interest Rate Contracts [Member] | Pay Fixed/Receive Floating Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 72,317	$ 63,038
Fair Value, Assets	$ 413	$ 860
Derivative Asset Average Remaining Maturity Period	4 years 4 months 28 days	4 years 2 months 12 days
Notional Value, Liabilities	$ 27,680	$ 25,093
Fair Value, Liabilities	$ 388	$ 602
Derivative Liability Average Remaining Maturity Period	4 years 4 months 13 days	5 years 9 months 3 days
Interest Rate Contracts [Member] | Options [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 32,509	$ 29,091
Fair Value, Assets	$ 46	$ 22
Derivative Asset Average Remaining Maturity Period	1 year 6 months 10 days	1 year 7 months 10 days
Notional Value, Liabilities	$ 1,960	$ 880
Fair Value, Liabilities	$ 22	$ 14
Derivative Liability Average Remaining Maturity Period	2 years 3 months 4 days	4 years 2 months 27 days
Interest Rate Contracts [Member] | Options [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 2,060	$ 880
Fair Value, Assets	$ 23	$ 15
Derivative Asset Average Remaining Maturity Period	2 years 4 months 17 days	4 years 2 months 27 days
Notional Value, Liabilities	$ 30,225	$ 27,056
Fair Value, Liabilities	$ 43	$ 20
Derivative Liability Average Remaining Maturity Period	1 year 4 months 20 days	1 year 6 months
Interest Rate Contracts [Member] | Futures [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Assets	$ 8,151	$ 7,007
Fair Value, Assets	$ 8	$ 4
Derivative Asset Average Remaining Maturity Period	1 year 29 days	1 year 2 months 16 days
Foreign Exchange Rate Contracts [Member] | Forwards, Spots and Swaps [Member]
Derivative [Line Items]
Notional Value, Assets	$ 26,646	$ 24,099
Fair Value, Assets	$ 776	$ 656
Derivative Asset Average Remaining Maturity Period	9 months 7 days	9 months 22 days
Notional Value, Liabilities	$ 25,751	$ 23,440
Fair Value, Liabilities	$ 756	$ 636
Derivative Liability Average Remaining Maturity Period	9 months 7 days	9 months 29 days
Foreign Exchange Option [Member] | Purchased [Member]
Derivative [Line Items]
Notional Value, Assets	$ 4,399	$ 4,026
Fair Value, Assets	$ 108	$ 83
Derivative Asset Average Remaining Maturity Period	1 year 15 days	1 year 2 months 12 days
Foreign Exchange Option [Member] | Written [Member]
Derivative [Line Items]
Notional Value, Liabilities	$ 4,399	$ 4,026
Fair Value, Liabilities	$ 108	$ 83
Derivative Liability Average Remaining Maturity Period	1 year 15 days	1 year 2 months 12 days