Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2016	Dec. 31, 2015
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, total, amortized cost		$ 66,276	$ 61,817
Available-for-sale securities	[1]	$ 67,155	$ 61,997
Available-for-sale securities, total, weighted-average yield		2.09%	2.21%
Available-for-sale securities, total, weighted-average maturity in years		4 years 2 months 12 days	4 years 8 months 12 days
Held-to-maturity securities, total, amortized cost		$ 42,873	$ 43,590
Held-to-maturity securities, total, weighted-average maturity in years		3 years 9 months 18 days	4 years 2 months 12 days
Held-to-maturity securities, total, fair value		$ 43,406	$ 43,493
Held-to-maturity securities, total, weighted-average yield		1.90%	1.92%
Amortized Cost of Investment Securities		$ 109,149	$ 105,407
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		$ 2,729
Available-for-sale securities, maturing after one year through five years, amortized cost		7,529
Available-for-sale securities, maturing after five years through ten years, amortized cost		4,195
Available-for-sale securities, maturing after ten years, amortized cost		1
Available-for-sale securities, total, amortized cost		14,454	$ 4,611
Available-for-sale securities, maturing in one year or less, fair value		2,731
Available-for-sale securities, maturing after one year through five years, fair value		7,575
Available-for-sale securities, maturing after five years through ten years, fair value		4,322
Available-for-sale securities, maturing after ten years, fair value		2
Available-for-sale securities		$ 14,630	4,596
Available-for-sale securities, maturing in one year or less, weighted-average yield		0.88%
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 yield		1.08%
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		2 years 10 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		1.89%
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		6 years 2 months 12 days
Available-for-sale securities, maturing after ten years, weighted-average yield		4.15%
Available-for-sale securities, maturing after ten years, weighted -average maturity in years		10 years 10 months 24 days
Available-for-sale securities, total, weighted-average yield		1.28%
Available-for-sale securities, total, weighted-average maturity in years		3 years 4 months 24 days
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 424
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, amortized cost		$ 734
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, maturing after five years through ten years, amortized cost		$ 3,609
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		6 years 8 months 12 days
Held-to-maturity securities, total, amortized cost		$ 4,767	2,925
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, fair value		$ 426
Held to maturity securities, maturing after one year through five years, fair value		750
Held-to-maturity securities, maturing after five years through ten years, fair value		3,686
Held-to-maturity securities, total, fair value		$ 4,862	2,919
Held-to-maturity securities, maturing in one year or less, weighted-average yield		0.97%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.76%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.81%
Held-to-maturity securities, total, weighted-average yield		1.73%
Amortized Cost of Investment Securities		$ 19,221	$ 7,536
Amortized Cost of Investment Securities, Percentage		17.60%	7.20%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 697
Available-for-sale securities, maturing after one year through five years, amortized cost		41,085
Available-for-sale securities, maturing after five years through ten years, amortized cost		3,489
Available-for-sale securities, maturing after ten years, amortized cost		97
Available-for-sale securities, total, amortized cost		45,368
Available-for-sale securities, maturing in one year or less, fair value		703
Available-for-sale securities, maturing after one year through five years, fair value		41,592
Available-for-sale securities, maturing after five years through ten years, fair value		3,522
Available-for-sale securities, maturing after ten years, fair value		97
Available-for-sale securities		$ 45,914
Available-for-sale securities, maturing in one year or less, weighted-average yield		2.39%
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 yield		1.84%
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 10 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		1.97%
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 yield		1.74%
Available-for-sale securities, maturing after ten years, weighted -average maturity in years		12 years 6 months
Available-for-sale securities, total, weighted-average yield		1.85%
Available-for-sale securities, total, weighted-average maturity in years		4 years 1 month 6 days
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 479
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, amortized cost		$ 35,952
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, amortized cost		$ 1,595
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		5 years 9 months 18 days
Held-to-maturity securities, maturing after ten years, amortized cost		$ 37
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		12 years 2 months 12 days
Held-to-maturity securities, total, amortized cost		$ 38,063
Held-to-maturity securities, total, weighted-average maturity in years		3 years 7 months 6 days
Held-to-maturity securities, maturing in one year or less, fair value		$ 481
Held to maturity securities, maturing after one year through five years, fair value		36,368
Held-to-maturity securities, maturing after five years through ten years, fair value		1,607
Held-to-maturity securities, maturing after ten years, fair value		37
Held-to-maturity securities, total, fair value		$ 38,493
Held-to-maturity securities, maturing in one year or less, weighted-average yield		2.30%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.94%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.55%
Held-to-maturity securities, maturing after ten years, weighted-average yield		1.42%
Held-to-maturity securities, total, weighted-average yield		1.92%
Amortized Cost of Investment Securities		$ 83,431	$ 91,265
Amortized Cost of Investment Securities, Percentage		76.40%	86.60%
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 11
Available-for-sale securities, maturing after one year through five years, amortized cost		294
Available-for-sale securities, maturing after five years through ten years, amortized cost		222
Available-for-sale securities, total, amortized cost		527
Available-for-sale securities, maturing in one year or less, fair value		13
Available-for-sale securities, maturing after one year through five years, fair value		298
Available-for-sale securities, maturing after five years through ten years, fair value		226
Available-for-sale securities		$ 537
Available-for-sale securities, maturing in one year or less, weighted-average yield		6.96%
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 yield		3.19%
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		4 years
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.78%
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		5 years 7 months 6 days
Available-for-sale securities, total, weighted-average yield		3.10%
Available-for-sale 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 maturity in years		1 month 6 days
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 4
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		2 years 8 months 12 days
Held-to-maturity securities, maturing after five years through ten years, amortized cost		$ 4
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		5 years 10 months 24 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		17 years 7 months 6 days
Held-to-maturity securities, total, amortized cost		$ 8
Held-to-maturity securities, total, weighted-average maturity in years		4 years 2 months 12 days
Held to maturity securities, maturing after one year through five years, fair value		$ 7
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 yield		1.17%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.25%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.20%
Held-to-maturity securities, maturing after ten years, weighted-average yield		1.29%
Held-to-maturity securities, total, weighted-average yield		1.23%
Amortized Cost of Investment Securities		$ 535	$ 558
Amortized Cost of Investment Securities, Percentage		0.50%	0.50%
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		$ 2,175
Available-for-sale securities, maturing after one year through five years, amortized cost		674
Available-for-sale securities, maturing after five years through ten years, amortized cost		1,743
Available-for-sale securities, maturing after ten years, amortized cost		640
Available-for-sale securities, total, amortized cost		5,232	$ 5,149
Available-for-sale securities, maturing in one year or less, fair value		2,206
Available-for-sale securities, maturing after one year through five years, fair value		713
Available-for-sale securities, maturing after five years through ten years, fair value		1,810
Available-for-sale securities, maturing after ten years, fair value		647
Available-for-sale securities		$ 5,376	5,316
Available-for-sale securities, maturing in one year or less, weighted-average yield		7.10%
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 yield		6.40%
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		2 years 4 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		5.17%
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 yield		5.11%
Available-for-sale securities, maturing after ten years, weighted -average maturity in years		19 years 6 months
Available-for-sale securities, total, weighted-average yield		6.12%
Available-for-sale 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 maturity in years		4 months 24 days
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 1
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		2 years 8 months 12 days
Held-to-maturity securities, maturing after five years through ten years, amortized cost		$ 7
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		9 years 1 month 6 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		10 years 2 months 12 days
Held-to-maturity securities, total, amortized cost		$ 8	8
Held-to-maturity securities, total, weighted-average maturity in years		8 years 7 months 6 days
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		7
Held-to-maturity securities, total, fair value		$ 8	8
Held-to-maturity securities, maturing in one year or less, weighted-average yield		7.58%
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		2.60%
Held-to-maturity securities, maturing after ten years, weighted-average yield		8.07%
Held-to-maturity securities, total, weighted-average yield		3.18%
Amortized Cost of Investment Securities		$ 5,240	$ 5,157
Amortized Cost of Investment Securities, Percentage		4.80%	4.90%
Other Debt Securities and Obligations of Foreign Governments [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after ten years, amortized cost		$ 628
Available-for-sale securities, total, amortized cost		628
Available-for-sale securities, maturing after ten years, fair value		579
Available-for-sale securities		$ 579
Available-for-sale securities, maturing after ten years, weighted-average yield		2.62%
Available-for-sale securities, maturing after ten years, weighted -average maturity in years		15 years 9 months 18 days
Available-for-sale securities, total, weighted-average yield		2.62%
Available-for-sale securities, total, weighted-average maturity in years		15 years 9 months 18 days
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 6
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		4 months 24 days
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 21
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 8 months 12 days
Held-to-maturity securities, total, amortized cost		$ 27
Held-to-maturity securities, total, weighted-average maturity in years		3 years
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		21
Held-to-maturity securities, total, fair value		$ 27
Held-to-maturity securities, maturing in one year or less, weighted-average yield		2.00%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.55%
Held-to-maturity securities, total, weighted-average yield		1.65%
Amortized Cost of Investment Securities		$ 722	$ 891
Amortized Cost of Investment Securities, Percentage		0.70%	0.80%
Other Investments Including Perpetual Preferred Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, total, amortized cost		$ 67
Available-for-sale securities		$ 119
Available-for-sale securities, total, weighted-average yield		5.89%
Available-for-sale securities, total, weighted-average maturity in years		5 years 2 months 12 days

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