Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2017	Dec. 31, 2016
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, total, amortized cost		$ 67,634	$ 66,985
Available-for-sale securities, total, fair value	[1]	$ 67,031	$ 66,284
Available-for-sale securities, total, weighted-average maturity in years		4 years 10 months 24 days	5 years 1 month 6 days
Available-for-sale securities, total, weighted-average yield		2.12%	2.06%
Held-to-maturity securities, total, amortized cost		$ 43,393	$ 42,991
Held-to-maturity securities, total, fair value		$ 42,906	$ 42,435
Held-to-maturity securities, total, weighted-average maturity in years		4 years 7 months 6 days	4 years 7 months 6 days
Held-to-maturity securities, total, weighted-average yield		2.03%	1.93%
Amortized Cost of Investment Securities		$ 111,027	$ 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		$ 4,503
Available-for-sale securities, maturing after one year through five years, amortized cost		10,730
Available-for-sale securities, maturing after five years through ten years, amortized cost		4,075
Available-for-sale securities, maturing after ten years, amortized cost		1
Available-for-sale securities, total, amortized cost		19,309	$ 17,314
Available-for-sale securities, maturing in one year or less, fair value		4,500
Available-for-sale securities, maturing after one year through five years, fair value		10,638
Available-for-sale securities, maturing after five years through ten years, fair value		4,014
Available-for-sale securities, maturing after ten years, fair value		1
Available-for-sale securities, total, fair value		$ 19,153	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		2 years 9 months 18 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		5 years 10 months 24 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		10 years 4 months 24 days
Available-for-sale securities, total, weighted-average maturity in years		2 years 10 months 24 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		0.84%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		1.25%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		1.90%
Available-for-sale securities, maturing after ten years, weighted-average yield		4.15%
Available-for-sale securities, total, weighted-average yield		1.29%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 350
Held-to-maturity securities, maturing after one year through five years, amortized cost		705
Held-to-maturity securities, maturing after five years through ten years, amortized cost		4,521
Held-to-maturity securities, total, amortized cost		5,576	5,246
Held-to-maturity securities, maturing in one year or less, fair value		350
Held-to-maturity securities, maturing after one year through five years, fair value		711
Held-to-maturity securities, maturing after five years through ten years, fair value		4,416
Held-to-maturity securities, total, fair value		$ 5,477	5,126
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		3 months 18 days
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		2 years 4 months 24 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		6 years 6 months
Held-to-maturity securities, total, weighted-average maturity in years		5 years 7 months 6 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.00%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.78%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.82%
Held-to-maturity securities, total, weighted-average yield		1.76%
Amortized Cost of Investment Securities		$ 24,885	$ 22,560
Amortized Cost of Investment Securities, Percentage		22.40%	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		$ 144
Available-for-sale securities, maturing after one year through five years, amortized cost		19,242
Available-for-sale securities, maturing after five years through ten years, amortized cost		20,931
Available-for-sale securities, maturing after ten years, amortized cost		2,189
Available-for-sale securities, total, amortized cost		42,506
Available-for-sale securities, maturing in one year or less, fair value		149
Available-for-sale securities, maturing after one year through five years, fair value		19,212
Available-for-sale securities, maturing after five years through ten years, fair value		20,584
Available-for-sale securities, maturing after ten years, fair value		2,195
Available-for-sale securities, total, fair value		$ 42,140
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		8 months 12 days
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		4 years 2 months 12 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		5 years 9 months 18 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		12 years 1 month 6 days
Available-for-sale securities, total, weighted-average maturity in years		5 years 4 months 24 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		4.19%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		2.10%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		1.94%
Available-for-sale securities, maturing after ten years, weighted-average yield		1.89%
Available-for-sale securities, total, weighted-average yield		2.02%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 221
Held-to-maturity securities, maturing after one year through five years, amortized cost		25,002
Held-to-maturity securities, maturing after five years through ten years, amortized cost		12,313
Held-to-maturity securities, maturing after ten years, amortized cost		239
Held-to-maturity securities, total, amortized cost		37,775
Held-to-maturity securities, maturing in one year or less, fair value		223
Held-to-maturity securities, maturing after one year through five years, fair value		24,828
Held-to-maturity securities, maturing after five years through ten years, fair value		12,087
Held-to-maturity securities, maturing after ten years, fair value		240
Held-to-maturity securities, total, fair value		$ 37,378
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		7 months 6 days
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		11 years 4 months 24 days
Held-to-maturity securities, total, weighted-average maturity in years		4 years 4 months 24 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		2.93%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.08%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.04%
Held-to-maturity securities, maturing after ten years, weighted-average yield		1.80%
Held-to-maturity securities, total, weighted-average yield		2.07%
Amortized Cost of Investment Securities		$ 80,281	$ 81,698
Amortized Cost of Investment Securities, Percentage		72.30%	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		$ 351
Available-for-sale securities, maturing after five years through ten years, amortized cost		85
Available-for-sale securities, total, amortized cost		436
Available-for-sale securities, maturing after one year through five years, fair value		356
Available-for-sale securities, maturing after five years through ten years, fair value		87
Available-for-sale securities, total, fair value		$ 443
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		5 years 8 months 12 days
Available-for-sale securities, total, weighted-average maturity in years		3 years 6 months
Available-for-sale securities, maturing after one year through five years, weighted-average yield		4.33%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.64%
Available-for-sale securities, total, weighted-average yield		4.00%
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		3
Held-to-maturity securities, total, amortized cost		8
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		4
Held-to-maturity securities, maturing after ten years, fair value		5
Held-to-maturity securities, total, fair value		$ 16
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		2 years 7 months 6 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		6 years 9 months 18 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		17 years 1 month 6 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		1.88%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.60%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.75%
Held-to-maturity securities, maturing after ten years, weighted-average yield		1.61%
Held-to-maturity securities, total, weighted-average yield		1.67%
Amortized Cost of Investment Securities		$ 444	$ 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		$ 1,275
Available-for-sale securities, maturing after one year through five years, amortized cost		476
Available-for-sale securities, maturing after five years through ten years, amortized cost		2,246
Available-for-sale securities, maturing after ten years, amortized cost		1,356
Available-for-sale securities, total, amortized cost		5,353	$ 5,167
Available-for-sale securities, maturing in one year or less, fair value		1,287
Available-for-sale securities, maturing after one year through five years, fair value		501
Available-for-sale securities, maturing after five years through ten years, fair value		2,218
Available-for-sale securities, maturing after ten years, fair value		1,250
Available-for-sale securities, total, fair value		$ 5,256	5,039
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		3 months 18 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		8 years 6 months
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		8 years 8 months 12 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		7.11%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		6.23%
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		5.07%
Available-for-sale securities, total, weighted-average yield		5.80%
Held-to-maturity securities, maturing after five years through ten years, amortized cost		$ 6
Held-to-maturity securities, total, amortized cost		6	6
Held-to-maturity securities, maturing after five years through ten years, fair value		7
Held-to-maturity securities, total, fair value		$ 7	7
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		2 years 2 months 12 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		8 years 7 months 6 days
Held-to-maturity securities, total, weighted-average maturity in years		8 years 2 months 12 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		8.21%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		8.17%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		3.28%
Held-to-maturity securities, total, weighted-average yield		3.55%
Amortized Cost of Investment Securities		$ 5,359	$ 5,173
Amortized Cost of Investment Securities, Percentage		4.80%	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		$ 6
Held-to-maturity securities, maturing after one year through five years, amortized cost		22
Held-to-maturity securities, total, amortized cost		28
Held-to-maturity securities, maturing in one year or less, fair value		6
Held-to-maturity securities, maturing after one year through five years, fair value		22
Held-to-maturity securities, total, fair value		$ 28
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 3 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		2.10%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.90%
Held-to-maturity securities, total, weighted-average yield		1.94%
Amortized Cost of Investment Securities		$ 58	$ 62
Amortized Cost of Investment Securities, Percentage		0.10%	0.10%
Other Investment [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, total, amortized cost		$ 30	$ 27
Available-for-sale securities, total, fair value		$ 39	$ 36

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