Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 68,486	$ 68,717
Amortized Cost of Investment Securities		$ 113,098	$ 113,079
Amortized Cost of Investment Securities, Percentage		100.00%	100.00%
Available-for-sale securities, total, fair value	[1]	$ 67,125	$ 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		2.29%	2.25%
Held-to-maturity securities, Amortized Cost		$ 44,612	$ 44,362
Held-to-maturity securities, total, fair value		$ 43,408	$ 43,723
Held-to-maturity securities, total, weighted-average maturity in years		5 years 2 months 12 days	4 years 8 months 12 days
Held-to-maturity securities, total, weighted-average yield		2.24%	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		$ 4,338
Available-for-sale securities, maturing after one year through five years, amortized cost		17,042
Available-for-sale securities, maturing after five years through ten years, amortized cost		1,043
Available-for-sale securities, Amortized Cost		22,423	$ 23,586
Available-for-sale securities, maturing in one year or less, fair value		4,320
Amortized Cost of Investment Securities		27,576	$ 28,767
Available-for-sale securities, maturing after one year through five years, fair value		$ 16,640
Amortized Cost of Investment Securities, Percentage		24.40%	25.50%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 1,006
Available-for-sale securities, total, fair value		$ 21,966	$ 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 3 months 19 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		7 years
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		0.88%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		1.66%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.29%
Available-for-sale securities, total, weighted-average yield		1.54%
Held-to-maturity securities, maturing after one year through five years, amortized cost		$ 1,880
Held-to-maturity securities, maturing after five years through ten years, amortized cost		3,273
Held-to-maturity securities, Amortized Cost		5,153	5,181
Held-to-maturity securities, maturing after one year through five years, fair value		1,843
Held-to-maturity securities, maturing after five years through ten years, fair value		3,131
Held-to-maturity securities, total, fair value		$ 4,974	5,066
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		5 years 8 months 12 days
Held-to-maturity securities, total, weighted-average maturity in years		4 years 9 months 18 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.82%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		1.79%
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		$ 110
Available-for-sale securities, maturing after one year through five years, amortized cost		13,809
Available-for-sale securities, maturing after five years through ten years, amortized cost		22,800
Available-for-sale securities, maturing after ten years, amortized cost		2,556
Available-for-sale securities, Amortized Cost		39,275
Available-for-sale securities, maturing in one year or less, fair value		111
Amortized Cost of Investment Securities		78,701	$ 77,606
Available-for-sale securities, maturing after one year through five years, fair value		$ 13,498
Amortized Cost of Investment Securities, Percentage		69.60%	68.60%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 22,275
Available-for-sale securities, maturing after ten years, fair value		2,566
Available-for-sale securities, total, fair value		$ 38,450
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 6 months
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		6 years
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		14 years 6 months
Available-for-sale securities, total, weighted-average maturity in years		6 years
Available-for-sale securities, maturing in one year or less, weighted-average yield		4.22%
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		2.35%
Available-for-sale securities, maturing after ten years, weighted-average yield		2.78%
Available-for-sale securities, total, weighted-average yield		2.34%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 30
Held-to-maturity securities, maturing after one year through five years, amortized cost		18,232
Held-to-maturity securities, maturing after five years through ten years, amortized cost		20,867
Held-to-maturity securities, maturing after ten years, amortized cost		297
Held-to-maturity securities, Amortized Cost		39,426
Held-to-maturity securities, maturing in one year or less, fair value		30
Held-to-maturity securities, maturing after one year through five years, fair value		17,747
Held-to-maturity securities, maturing after five years through ten years, fair value		20,322
Held-to-maturity securities, maturing after ten years, fair value		298
Held-to-maturity securities, total, fair value		$ 38,397
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		1 year 3 months 19 days
Held-to-maturity securities, maturing after one year through five years, weighted-average maturity in years		4 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		13 years 10 months 25 days
Held-to-maturity securities, total, weighted-average maturity in years		5 years 3 months 19 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		2.54%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.14%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.42%
Held-to-maturity securities, maturing after ten years, weighted-average yield		2.67%
Held-to-maturity securities, total, weighted-average yield		2.29%
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after one year through five years, amortized cost		$ 408
Available-for-sale securities, Amortized Cost		408
Amortized Cost of Investment Securities		414	$ 419
Available-for-sale securities, maturing after one year through five years, fair value		$ 415
Amortized Cost of Investment Securities, Percentage		0.40%	0.40%
Available-for-sale securities, total, fair value		$ 415
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 8 months 12 days
Available-for-sale securities, total, weighted-average maturity in years		3 years 8 months 12 days
Available-for-sale securities, maturing after one year through five years, weighted-average yield		4.63%
Available-for-sale securities, total, weighted-average yield		4.63%
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		2
Held-to-maturity securities, Amortized Cost		6
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		4
Held-to-maturity securities, maturing after five years through ten years, fair value		2
Held-to-maturity securities, maturing after ten years, fair value		2
Held-to-maturity securities, total, fair value		$ 9
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, weighted-average maturity in years		1 year 8 months 12 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		4 years 8 months 12 days
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		13 years 3 months 19 days
Held-to-maturity securities, total, weighted-average maturity in years		2 years 10 months 25 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		2.48%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.60%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.56%
Held-to-maturity securities, maturing after ten years, weighted-average yield		2.50%
Held-to-maturity securities, total, weighted-average yield		2.58%
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		$ 129
Available-for-sale securities, maturing after one year through five years, amortized cost		661
Available-for-sale securities, maturing after five years through ten years, amortized cost		3,672
Available-for-sale securities, maturing after ten years, amortized cost		1,918
Available-for-sale securities, Amortized Cost		6,380	$ 6,240
Available-for-sale securities, maturing in one year or less, fair value		131
Amortized Cost of Investment Securities		6,386	$ 6,246
Available-for-sale securities, maturing after one year through five years, fair value		$ 678
Amortized Cost of Investment Securities, Percentage		5.60%	5.50%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 3,667
Available-for-sale securities, maturing after ten years, fair value		1,818
Available-for-sale securities, total, fair value		$ 6,294	$ 6,358
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		8 years 6 months
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		19 years 2 months 12 days
Available-for-sale securities, total, weighted-average maturity in years		11 years
Available-for-sale securities, maturing in one year or less, weighted-average yield		5.88%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		4.92%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		4.46%
Available-for-sale securities, maturing after ten years, weighted-average yield		4.11%
Available-for-sale securities, total, weighted-average yield		4.43%
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		8 months 12 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		8 years
Held-to-maturity securities, total, weighted-average maturity in years		7 years 4 months 24 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		6.33%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		6.78%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.18%
Held-to-maturity securities, total, weighted-average yield		2.75%
Other Debt Securities and Obligations of Foreign Governments [Member]
Contractual Maturities of Investment Securities [Line Items]
Amortized Cost of Investment Securities		$ 21	$ 41
Available-for-sale securities, maturing in one year or less, weighted-average yield		0.01%
Available-for-sale securities, total, weighted-average yield		0.01%
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
Held-to-maturity securities, total, weighted-average maturity in years		1 year
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.52%
Held-to-maturity securities, total, weighted-average yield		2.52%

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