Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 68,041	$ 66,985
Available-for-sale securities, total, fair value	[1]	$ 67,772	$ 66,284
Available-for-sale securities, total, weighted-average maturity in years		5 years 1 month 6 days	5 years 1 month 6 days
Available-for-sale securities, total, weighted-average yield		2.15%	2.06%
Held-to-maturity securities, Amortized Cost		$ 44,018	$ 42,991
Held-to-maturity securities, total, fair value		$ 43,758	$ 42,435
Held-to-maturity securities, total, weighted-average maturity in years		4 years 8 months 12 days	4 years 7 months 6 days
Held-to-maturity securities, total, weighted-average yield		2.09%	1.93%
Amortized Cost of Investment Securities		$ 112,059	$ 109,976
Amortized Cost of Investment Securities, Percentage		100.00%	100.00%
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		$ 5,188
Available-for-sale securities, maturing after one year through five years, amortized cost		13,124
Available-for-sale securities, maturing after five years through ten years, amortized cost		3,851
Available-for-sale securities, Amortized Cost		22,163	$ 17,314
Available-for-sale securities, maturing in one year or less, fair value		5,177
Available-for-sale securities, maturing after one year through five years, fair value		13,066
Available-for-sale securities, maturing after five years through ten years, fair value		3,825
Available-for-sale securities, total, fair value		$ 22,068	17,127
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		6 months
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 1 month 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		5 years 6 months
Available-for-sale securities, total, weighted-average maturity in years		2 years 10 months 25 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		0.82%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		1.51%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		1.86%
Available-for-sale securities, total, weighted-average yield		1.41%
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 1,546
Held-to-maturity securities, maturing after five years through ten years, amortized cost		3,647
Held-to-maturity securities, Amortized Cost		5,193	5,246
Held-to-maturity securities, maturing after one year through five years, fair value		1,548
Held-to-maturity securities, maturing after five years through ten years, fair value		3,590
Held-to-maturity securities, total, fair value		$ 5,138	5,126
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 6 months
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		6 years 2 months 12 days
Held-to-maturity securities, total, weighted-average maturity in years		5 years 4 months 24 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.80%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.81%
Held-to-maturity securities, total, weighted-average yield		1.81%
Amortized Cost of Investment Securities		$ 27,356	$ 22,560
Amortized Cost of Investment Securities, Percentage		24.40%	20.50%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 99
Available-for-sale securities, maturing after one year through five years, amortized cost		17,946
Available-for-sale securities, maturing after five years through ten years, amortized cost		19,291
Available-for-sale securities, maturing after ten years, amortized cost		2,416
Available-for-sale securities, Amortized Cost		39,752
Available-for-sale securities, maturing in one year or less, fair value		101
Available-for-sale securities, maturing after one year through five years, fair value		17,889
Available-for-sale securities, maturing after five years through ten years, fair value		19,149
Available-for-sale securities, maturing after ten years, fair value		2,422
Available-for-sale securities, total, fair value		$ 39,561
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		4 years 4 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		5 years 10 months 25 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		12 years 10 months 25 days
Available-for-sale securities, total, weighted-average maturity in years		5 years 7 months 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		4.27%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		2.03%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.11%
Available-for-sale securities, maturing after ten years, weighted-average yield		2.26%
Available-for-sale securities, total, weighted-average yield		2.09%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 129
Held-to-maturity securities, maturing after one year through five years, amortized cost		23,627
Held-to-maturity securities, maturing after five years through ten years, amortized cost		14,745
Held-to-maturity securities, maturing after ten years, amortized cost		287
Held-to-maturity securities, Amortized Cost		38,788
Held-to-maturity securities, maturing in one year or less, fair value		129
Held-to-maturity securities, maturing after one year through five years, fair value		23,525
Held-to-maturity securities, maturing after five years through ten years, fair value		14,635
Held-to-maturity securities, maturing after ten years, fair value		288
Held-to-maturity securities, total, fair value		$ 38,577
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		6 months
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 9 months 18 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, maturing after ten years, weighted-average maturity in years		12 years 4 months 24 days
Held-to-maturity securities, total, weighted-average maturity in years		4 years 7 months 6 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		3.08%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.07%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.21%
Held-to-maturity securities, maturing after ten years, weighted-average yield		2.21%
Held-to-maturity securities, total, weighted-average yield		2.13%
Amortized Cost of Investment Securities		$ 78,540	$ 81,698
Amortized Cost of Investment Securities, Percentage		70.10%	74.30%
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after one year through five years, amortized cost		$ 333
Available-for-sale securities, maturing after five years through ten years, amortized cost		85
Available-for-sale securities, Amortized Cost		418
Available-for-sale securities, maturing after one year through five years, fair value		338
Available-for-sale securities, maturing after five years through ten years, fair value		87
Available-for-sale securities, total, fair value		$ 425
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 9 months 18 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		5 years 3 months 19 days
Available-for-sale securities, total, weighted-average maturity in years		4 years 1 month 6 days
Available-for-sale securities, maturing after one year through five years, weighted-average yield		0.95%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.92%
Available-for-sale securities, total, weighted-average yield		1.35%
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 5
Held-to-maturity securities, maturing after five years through ten years, amortized cost		2
Held-to-maturity securities, Amortized Cost		7
Held-to-maturity securities, maturing in one year or less, fair value		1
Held-to-maturity securities, maturing after one year through five years, fair value		6
Held-to-maturity securities, maturing after five years through ten years, fair value		2
Held-to-maturity securities, maturing after ten years, fair value		3
Held-to-maturity securities, total, fair value		$ 12
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		6 months
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 2 months 12 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		6 years 1 month 6 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		16 years 7 months 6 days
Held-to-maturity securities, total, weighted-average maturity in years		3 years 10 months 25 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.88%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.91%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.02%
Held-to-maturity securities, maturing after ten years, weighted-average yield		1.85%
Held-to-maturity securities, total, weighted-average yield		1.94%
Amortized Cost of Investment Securities		$ 425	$ 483
Amortized Cost of Investment Securities, Percentage		0.40%	0.40%
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		$ 367
Available-for-sale securities, maturing after one year through five years, amortized cost		580
Available-for-sale securities, maturing after five years through ten years, amortized cost		3,519
Available-for-sale securities, maturing after ten years, amortized cost		1,215
Available-for-sale securities, Amortized Cost		5,681	$ 5,167
Available-for-sale securities, maturing in one year or less, fair value		369
Available-for-sale securities, maturing after one year through five years, fair value		608
Available-for-sale securities, maturing after five years through ten years, fair value		3,548
Available-for-sale securities, maturing after ten years, fair value		1,156
Available-for-sale securities, total, fair value		$ 5,681	5,039
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		2 months 12 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 2 months 12 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		8 years 8 months 12 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		19 years 10 months 25 days
Available-for-sale securities, total, weighted-average maturity in years		10 years
Available-for-sale securities, maturing in one year or less, weighted-average yield		7.37%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		6.04%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		5.41%
Available-for-sale securities, maturing after ten years, weighted-average yield		4.94%
Available-for-sale securities, total, weighted-average yield		5.50%
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 after one year through five years, weighted-average maturity in years		3 years 3 months 19 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		8 years 3 months 19 days
Held-to-maturity securities, total, weighted-average maturity in years		7 years 9 months 18 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		8.15%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.77%
Held-to-maturity securities, total, weighted-average yield		3.32%
Amortized Cost of Investment Securities		$ 5,687	$ 5,173
Amortized Cost of Investment Securities, Percentage		5.10%	4.70%
Other Debt Securities and Obligations of Foreign Governments [Member]
Contractual Maturities of Investment Securities [Line Items]
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 2
Held-to-maturity securities, maturing after one year through five years, amortized cost		22
Held-to-maturity securities, Amortized Cost		24
Held-to-maturity securities, maturing in one year or less, fair value		2
Held-to-maturity securities, maturing after one year through five years, fair value		22
Held-to-maturity securities, total, fair value		$ 24
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		2 years 9 months 18 days
Held-to-maturity securities, total, weighted-average maturity in years		2 years 7 months 6 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.68%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.09%
Held-to-maturity securities, total, weighted-average yield		2.05%
Amortized Cost of Investment Securities		$ 51	$ 62
Amortized Cost of Investment Securities, Percentage			0.10%
Other Investment [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		27	$ 27
Available-for-sale securities, total, fair value		$ 37	$ 36
Available-for-sale securities, total, weighted-average yield		0.01%

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