Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 67,930	$ 68,717
Amortized Cost of Investment Securities		$ 113,985	$ 113,079
Amortized Cost of Investment Securities, Percentage		100.00%	100.00%
Available-for-sale securities, total, fair value	[1]	$ 66,347	$ 68,137
Available-for-sale securities, total, weighted-average maturity in years		5 years 6 months	5 years 1 month 6 days
Available-for-sale securities, total, weighted-average yield		2.38%	2.25%
Held-to-maturity securities, Amortized Cost		$ 46,055	$ 44,362
Held-to-maturity securities, total, fair value		$ 44,617	$ 43,723
Held-to-maturity securities, total, weighted-average maturity in years		5 years 3 months 19 days	4 years 8 months 12 days
Held-to-maturity securities, total, weighted-average yield		2.32%	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		$ 4,105
Available-for-sale securities, maturing after one year through five years, amortized cost		16,109
Available-for-sale securities, maturing after five years through ten years, amortized cost		1,030
Available-for-sale securities, Amortized Cost		21,244	$ 23,586
Available-for-sale securities, maturing in one year or less, fair value		4,088
Amortized Cost of Investment Securities		26,387	$ 28,767
Available-for-sale securities, maturing after one year through five years, fair value		$ 15,658
Amortized Cost of Investment Securities, Percentage		23.20%	25.50%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 985
Available-for-sale securities, total, fair value		$ 20,731	$ 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 2 months 12 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		6 years 9 months 18 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		1.05%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		1.69%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.28%
Available-for-sale securities, total, weighted-average yield		1.59%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 276
Held-to-maturity securities, maturing after one year through five years, amortized cost		2,405
Held-to-maturity securities, maturing after five years through ten years, amortized cost		2,462
Held-to-maturity securities, Amortized Cost		5,143	5,181
Held-to-maturity securities, maturing in one year or less, fair value		274
Held-to-maturity securities, maturing after one year through five years, fair value		2,321
Held-to-maturity securities, maturing after five years through ten years, fair value		2,346
Held-to-maturity securities, total, fair value		$ 4,941	5,066
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		3 years 10 months 25 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		5 years 7 months 6 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		1.74%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.75%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.85%
Held-to-maturity securities, total, weighted-average yield		1.80%
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		17,048
Available-for-sale securities, maturing after five years through ten years, amortized cost		19,669
Available-for-sale securities, maturing after ten years, amortized cost		2,599
Available-for-sale securities, Amortized Cost		39,415
Available-for-sale securities, maturing in one year or less, fair value		100
Amortized Cost of Investment Securities		80,295	$ 77,606
Available-for-sale securities, maturing after one year through five years, fair value		$ 16,518
Amortized Cost of Investment Securities, Percentage		70.40%	68.60%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 19,196
Available-for-sale securities, maturing after ten years, fair value		2,612
Available-for-sale securities, total, fair value		$ 38,426
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		4 years 7 months 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		6 years 3 months 19 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 1 month 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		4.23%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		2.22%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.53%
Available-for-sale securities, maturing after ten years, weighted-average yield		3.06%
Available-for-sale securities, total, weighted-average yield		2.43%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 47
Held-to-maturity securities, maturing after one year through five years, amortized cost		21,758
Held-to-maturity securities, maturing after five years through ten years, amortized cost		18,768
Held-to-maturity securities, maturing after ten years, amortized cost		307
Held-to-maturity securities, Amortized Cost		40,880
Held-to-maturity securities, maturing in one year or less, fair value		47
Held-to-maturity securities, maturing after one year through five years, fair value		21,032
Held-to-maturity securities, maturing after five years through ten years, fair value		18,250
Held-to-maturity securities, maturing after ten years, fair value		308
Held-to-maturity securities, total, fair value		$ 39,637
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		10 months 25 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		6 years 4 months 24 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		13 years 9 months 18 days
Held-to-maturity securities, total, weighted-average maturity in years		5 years 4 months 24 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		2.40%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.16%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.64%
Held-to-maturity securities, maturing after ten years, weighted-average yield		2.97%
Held-to-maturity securities, total, weighted-average yield		2.39%
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after one year through five years, amortized cost		$ 405
Available-for-sale securities, Amortized Cost		405
Amortized Cost of Investment Securities		410	$ 419
Available-for-sale securities, maturing after one year through five years, fair value		$ 411
Amortized Cost of Investment Securities, Percentage		0.40%	0.40%
Available-for-sale securities, total, fair value		$ 411
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		4.73%
Available-for-sale securities, total, weighted-average yield		4.73%
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 4
Held-to-maturity securities, maturing after five years through ten years, amortized cost		1
Held-to-maturity securities, Amortized Cost		5
Held-to-maturity securities, maturing after one year through five years, fair value		5
Held-to-maturity securities, maturing after five years through ten years, fair value		1
Held-to-maturity securities, maturing after ten years, fair value		5
Held-to-maturity securities, total, fair value		$ 11
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		3 years 8 months 12 days
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		4 years 8 months 12 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		4 years 6 months
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		14 years 8 months 12 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		2.80%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.80%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.87%
Held-to-maturity securities, maturing after ten years, weighted-average yield		2.84%
Held-to-maturity securities, total, weighted-average yield		2.82%
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		$ 147
Available-for-sale securities, maturing after one year through five years, amortized cost		683
Available-for-sale securities, maturing after five years through ten years, amortized cost		4,121
Available-for-sale securities, maturing after ten years, amortized cost		1,915
Available-for-sale securities, Amortized Cost		6,866	$ 6,240
Available-for-sale securities, maturing in one year or less, fair value		149
Amortized Cost of Investment Securities		6,872	$ 6,246
Available-for-sale securities, maturing after one year through five years, fair value		$ 697
Amortized Cost of Investment Securities, Percentage		6.00%	5.50%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 4,115
Available-for-sale securities, maturing after ten years, fair value		1,818
Available-for-sale securities, total, fair value		$ 6,779	$ 6,358
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 2 months 12 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		8 years 4 months 24 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		18 years 10 months 25 days
Available-for-sale securities, total, weighted-average maturity in years		10 years 7 months 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		5.82%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		4.84%
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		4.08%
Available-for-sale securities, total, weighted-average yield		4.36%
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		6 months
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		3 years 7 months 6 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		7 years 8 months 12 days
Held-to-maturity securities, total, weighted-average maturity in years		7 years 2 months 12 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		6.27%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		6.72%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.02%
Held-to-maturity securities, total, weighted-average yield		2.59%
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 after one year through five years, amortized cost		21
Held-to-maturity securities, Amortized Cost		21
Held-to-maturity securities, maturing after one year through five years, fair value		21
Held-to-maturity securities, total, fair value		$ 21
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		1 year 3 months 19 days
Held-to-maturity securities, total, weighted-average maturity in years		1 year 3 months 19 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		3.15%
Held-to-maturity securities, total, weighted-average yield		3.15%

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