Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 66,916	$ 68,717
Amortized Cost of Investment Securities		$ 112,962	$ 113,079
Amortized Cost of Investment Securities, Percentage		100.00%	100.00%
Available-for-sale securities, total, fair value	[1]	$ 64,912	$ 68,137
Available-for-sale securities, total, weighted-average maturity in years		5 years 7 months 6 days	5 years 1 month 6 days
Available-for-sale securities, total, weighted-average yield		2.43%	2.25%
Held-to-maturity securities, Amortized Cost		$ 46,046	$ 44,362
Held-to-maturity securities, total, fair value		$ 44,332	$ 43,723
Held-to-maturity securities, total, weighted-average maturity in years		5 years 4 months 24 days	4 years 8 months 12 days
Held-to-maturity securities, total, weighted-average yield		2.36%	2.14%
U.S. Treasury and Agencies [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 3,028
Available-for-sale securities, maturing after one year through five years, amortized cost		16,490
Available-for-sale securities, maturing after five years through ten years, amortized cost		658
Available-for-sale securities, Amortized Cost		20,176	$ 23,586
Available-for-sale securities, maturing in one year or less, fair value		3,016
Amortized Cost of Investment Securities		25,288	$ 28,767
Available-for-sale securities, maturing after one year through five years, fair value		$ 15,935
Amortized Cost of Investment Securities, Percentage		22.40%	25.50%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 630
Available-for-sale securities, total, fair value		$ 19,581	$ 23,301
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		4 months 24 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		7 years 7 months 6 days
Available-for-sale securities, total, weighted-average maturity in years		2 years 9 months 18 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		1.26%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		1.70%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.85%
Available-for-sale securities, total, weighted-average yield		1.67%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 650
Held-to-maturity securities, maturing after one year through five years, amortized cost		2,912
Held-to-maturity securities, maturing after five years through ten years, amortized cost		1,550
Held-to-maturity securities, Amortized Cost		5,112	5,181
Held-to-maturity securities, maturing in one year or less, fair value		646
Held-to-maturity securities, maturing after one year through five years, fair value		2,755
Held-to-maturity securities, maturing after five years through ten years, fair value		1,475
Held-to-maturity securities, total, fair value		$ 4,876	5,066
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		8 months 12 days
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		4 years 4 months 24 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		5 years 8 months 12 days
Held-to-maturity securities, total, weighted-average maturity in years		4 years 3 months 18 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.73%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.64%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.10%
Held-to-maturity securities, total, weighted-average yield		1.79%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 69
Available-for-sale securities, maturing after one year through five years, amortized cost		13,644
Available-for-sale securities, maturing after five years through ten years, amortized cost		23,195
Available-for-sale securities, maturing after ten years, amortized cost		2,583
Available-for-sale securities, Amortized Cost		39,491
Available-for-sale securities, maturing in one year or less, fair value		69
Amortized Cost of Investment Securities		80,393	$ 77,606
Available-for-sale securities, maturing after one year through five years, fair value		$ 13,074
Amortized Cost of Investment Securities, Percentage		71.20%	68.60%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 22,574
Available-for-sale securities, maturing after ten years, fair value		2,598
Available-for-sale securities, total, fair value		$ 38,315
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		4 months 24 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		4 years 4 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		6 years 3 months 18 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		14 years 4 months 24 days
Available-for-sale securities, total, weighted-average maturity in years		6 years 2 months 12 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		3.83%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		2.09%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.63%
Available-for-sale securities, maturing after ten years, weighted-average yield		3.18%
Available-for-sale securities, total, weighted-average yield		2.48%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 36
Held-to-maturity securities, maturing after one year through five years, amortized cost		15,158
Held-to-maturity securities, maturing after five years through ten years, amortized cost		25,348
Held-to-maturity securities, maturing after ten years, amortized cost		360
Held-to-maturity securities, Amortized Cost		40,902
Held-to-maturity securities, maturing in one year or less, fair value		36
Held-to-maturity securities, maturing after one year through five years, fair value		14,479
Held-to-maturity securities, maturing after five years through ten years, fair value		24,544
Held-to-maturity securities, maturing after ten years, fair value		361
Held-to-maturity securities, total, fair value		$ 39,420
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		9 months 18 days
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		4 years
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		6 years 3 months 18 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		14 years
Held-to-maturity securities, total, weighted-average maturity in years		5 years 6 months
Held-to-maturity securities, maturing in one year or less, weighted-average yield		2.51%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.03%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.67%
Held-to-maturity securities, maturing after ten years, weighted-average yield		3.06%
Held-to-maturity securities, total, weighted-average yield		2.43%
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after one year through five years, amortized cost		$ 402
Available-for-sale securities, Amortized Cost		402
Amortized Cost of Investment Securities		407	$ 419
Available-for-sale securities, maturing after one year through five years, fair value		$ 408
Amortized Cost of Investment Securities, Percentage		0.30%	0.40%
Available-for-sale securities, total, fair value		$ 408
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 7 months 6 days
Available-for-sale securities, total, weighted-average maturity in years		3 years 7 months 6 days
Available-for-sale securities, maturing after one year through five years, weighted-average yield		3.49%
Available-for-sale securities, total, weighted-average yield		3.49%
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 3
Held-to-maturity securities, maturing after five years through ten years, amortized cost		2
Held-to-maturity securities, Amortized Cost		5
Held-to-maturity securities, maturing after one year through five years, fair value		4
Held-to-maturity securities, maturing after five years through ten years, fair value		3
Held-to-maturity securities, maturing after ten years, fair value		1
Held-to-maturity securities, total, fair value		$ 8
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 3 months 18 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		6 years 4 months 24 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		17 years
Held-to-maturity securities, total, weighted-average maturity in years		4 years 4 months 24 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.90%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		3.00%
Held-to-maturity securities, maturing after ten years, weighted-average yield		2.84%
Held-to-maturity securities, total, weighted-average yield		2.93%
Obligations of State and Political Subdivisions [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 238
Available-for-sale securities, maturing after one year through five years, amortized cost		583
Available-for-sale securities, maturing after five years through ten years, amortized cost		3,886
Available-for-sale securities, maturing after ten years, amortized cost		2,140
Available-for-sale securities, Amortized Cost		6,847	$ 6,240
Available-for-sale securities, maturing in one year or less, fair value		242
Amortized Cost of Investment Securities		6,853	$ 6,246
Available-for-sale securities, maturing after one year through five years, fair value		$ 587
Amortized Cost of Investment Securities, Percentage		6.10%	5.50%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 3,818
Available-for-sale securities, maturing after ten years, fair value		1,961
Available-for-sale securities, total, fair value		$ 6,608	$ 6,358
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		7 months 6 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 6 months
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		8 years 2 months 12 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		19 years
Available-for-sale securities, total, weighted-average maturity in years		10 years 10 months 24 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		5.70%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		4.62%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		4.37%
Available-for-sale securities, maturing after ten years, weighted-average yield		4.11%
Available-for-sale securities, total, weighted-average yield		4.35%
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 1
Held-to-maturity securities, maturing after five years through ten years, amortized cost		5
Held-to-maturity securities, Amortized Cost		6	6
Held-to-maturity securities, maturing after one year through five years, fair value		1
Held-to-maturity securities, maturing after five years through ten years, fair value		6
Held-to-maturity securities, total, fair value		$ 7	7
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		2 months 12 days
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 4 months 24 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		7 years 6 months
Held-to-maturity securities, total, weighted-average maturity in years		7 years
Held-to-maturity securities, maturing in one year or less, weighted-average yield		6.24%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		6.67%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.01%
Held-to-maturity securities, total, weighted-average yield		2.55%
Other Debt Securities and Obligations of Foreign Governments [Member]
Contractual Maturities of Investment Securities [Line Items]
Amortized Cost of Investment Securities		$ 21	$ 41
Held-to-maturity securities, maturing in one year or less, amortized cost		9
Held-to-maturity securities, maturing after one year through five years, amortized cost		12
Held-to-maturity securities, Amortized Cost		21
Held-to-maturity securities, maturing in one year or less, fair value		9
Held-to-maturity securities, maturing after one year through five years, fair value		12
Held-to-maturity securities, total, fair value		$ 21
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		1 month 6 days
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		1 year 2 months 12 days
Held-to-maturity securities, total, weighted-average maturity in years		8 months 12 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		3.21%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		3.08%
Held-to-maturity securities, total, weighted-average yield		3.14%

[1] Includes only collateral pledged by the Company where counterparties have the right to sell or pledge the collateral.