Investment Securities - Investment Securities (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2019	Dec. 31, 2018
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, Amortized Cost		$ 68,599	$ 67,381
Amortized Cost of Investment Securities		$ 114,884	$ 113,431
Amortized Cost of Investment Securities, Percentage		100.00%	100.00%
Available-for-sale securities, total, fair value	[1]	$ 68,113	$ 66,115
Available-for-sale securities, total, weighted-average maturity in years		4 years 8 months 12 days	5 years 4 months 24 days
Available-for-sale securities, total, weighted-average yield		2.62%	2.57%
Held-to-maturity securities, Amortized Cost		$ 46,285	$ 46,050
Held-to-maturity securities, total, fair value		$ 45,735	$ 44,964
Held-to-maturity securities, total, weighted-average maturity in years		4 years 6 months	5 years 2 months 12 days
Held-to-maturity securities, total, weighted-average yield		2.52%	2.46%
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		$ 2,064
Available-for-sale securities, maturing after one year through five years, amortized cost		16,534
Available-for-sale securities, maturing after five years through ten years, amortized cost		529
Available-for-sale securities, Amortized Cost		19,127	$ 19,604
Available-for-sale securities, maturing in one year or less, fair value		2,056
Amortized Cost of Investment Securities		24,058	$ 24,706
Available-for-sale securities, maturing after one year through five years, fair value		$ 16,339
Amortized Cost of Investment Securities, Percentage		20.90%	21.80%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 524
Available-for-sale securities, total, fair value		$ 18,919	$ 19,257
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		2 years 7 months 6 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		7 years 2 months 12 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		0 years
Available-for-sale securities, total, weighted-average maturity in years		2 years 6 months
Available-for-sale securities, maturing in one year or less, weighted-average yield		1.51%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		1.77%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.80%
Available-for-sale securities, total, weighted-average yield		1.77%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 500
Held-to-maturity securities, maturing after one year through five years, amortized cost		3,901
Held-to-maturity securities, maturing after five years through ten years, amortized cost		530
Held-to-maturity securities, Amortized Cost		4,931	5,102
Held-to-maturity securities, maturing in one year or less, fair value		498
Held-to-maturity securities, maturing after one year through five years, fair value		3,820
Held-to-maturity securities, maturing after five years through ten years, fair value		524
Held-to-maturity securities, total, fair value		$ 4,842	4,961
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		4 years 1 month 6 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, maturing after ten years, weighted-average maturity in years		0 years
Held-to-maturity securities, total, weighted-average maturity in years		4 years
Held-to-maturity securities, maturing in one year or less, weighted-average yield		1.72%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		1.71%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.45%
Held-to-maturity securities, total, weighted-average yield		1.79%
Mortgage-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing in one year or less, amortized cost		$ 19
Available-for-sale securities, maturing after one year through five years, amortized cost		25,789
Available-for-sale securities, maturing after five years through ten years, amortized cost		14,104
Available-for-sale securities, maturing after ten years, amortized cost		2,357
Available-for-sale securities, Amortized Cost		42,269
Available-for-sale securities, maturing in one year or less, fair value		19
Amortized Cost of Investment Securities		83,596	$ 81,464
Available-for-sale securities, maturing after one year through five years, fair value		$ 25,578
Amortized Cost of Investment Securities, Percentage		72.80%	71.80%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 13,923
Available-for-sale securities, maturing after ten years, fair value		2,361
Available-for-sale securities, total, fair value		$ 41,881
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		3 years 10 months 24 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		13 years 3 months 18 days
Available-for-sale securities, total, weighted-average maturity in years		5 years 1 month 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		3.10%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		2.66%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		2.70%
Available-for-sale securities, maturing after ten years, weighted-average yield		3.50%
Available-for-sale securities, total, weighted-average yield		2.72%
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 158
Held-to-maturity securities, maturing after one year through five years, amortized cost		26,001
Held-to-maturity securities, maturing after five years through ten years, amortized cost		14,860
Held-to-maturity securities, maturing after ten years, amortized cost		308
Held-to-maturity securities, Amortized Cost		41,327
Held-to-maturity securities, maturing in one year or less, fair value		156
Held-to-maturity securities, maturing after one year through five years, fair value		25,672
Held-to-maturity securities, maturing after five years through ten years, fair value		14,726
Held-to-maturity securities, maturing after ten years, fair value		307
Held-to-maturity securities, total, fair value		$ 40,861
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 9 months 18 days
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		12 years 10 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		2.08%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		2.49%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.79%
Held-to-maturity securities, maturing after ten years, weighted-average yield		3.44%
Held-to-maturity securities, total, weighted-average yield		2.61%
Asset-Backed Securities [Member]
Contractual Maturities of Investment Securities [Line Items]
Available-for-sale securities, maturing after one year through five years, amortized cost		$ 389
Available-for-sale securities, Amortized Cost		389
Amortized Cost of Investment Securities		394	$ 402
Available-for-sale securities, maturing after one year through five years, fair value		$ 395
Amortized Cost of Investment Securities, Percentage		0.40%	0.40%
Available-for-sale securities, total, fair value		$ 395
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		0 years
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		3 years 4 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		0 years
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		0 years
Available-for-sale securities, total, weighted-average maturity in years		3 years 4 months 24 days
Available-for-sale securities, maturing after one year through five years, weighted-average yield		3.77%
Available-for-sale securities, total, weighted-average yield		3.77%
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		8
Held-to-maturity securities, maturing after five years through ten years, fair value		1
Held-to-maturity securities, maturing after ten years, fair value		1
Held-to-maturity securities, total, fair value		$ 10
Held-to-maturity securities, maturing in one year or less, weighted-average maturity in years		0 years
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, 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		14 years 9 months 18 days
Held-to-maturity securities, total, weighted-average maturity in years		3 years 7 months 6 days
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		3.20%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		3.30%
Held-to-maturity securities, maturing after ten years, weighted-average yield		3.19%
Held-to-maturity securities, total, weighted-average yield		3.21%
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		$ 266
Available-for-sale securities, maturing after one year through five years, amortized cost		571
Available-for-sale securities, maturing after five years through ten years, amortized cost		5,192
Available-for-sale securities, maturing after ten years, amortized cost		785
Available-for-sale securities, Amortized Cost		6,814	$ 6,836
Available-for-sale securities, maturing in one year or less, fair value		268
Amortized Cost of Investment Securities		6,819	$ 6,842
Available-for-sale securities, maturing after one year through five years, fair value		$ 585
Amortized Cost of Investment Securities, Percentage		5.90%	6.00%
Available-for-sale securities, maturing after five years through ten years, fair value		$ 5,299
Available-for-sale securities, maturing after ten years, fair value		766
Available-for-sale securities, total, fair value		$ 6,918	$ 6,701
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 4 months 24 days
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		7 years 8 months 12 days
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		20 years 4 months 24 days
Available-for-sale securities, total, weighted-average maturity in years		8 years 7 months 6 days
Available-for-sale securities, maturing in one year or less, weighted-average yield		5.65%
Available-for-sale securities, maturing after one year through five years, weighted-average yield		4.53%
Available-for-sale securities, maturing after five years through ten years, weighted-average yield		4.30%
Available-for-sale securities, maturing after ten years, weighted-average yield		4.04%
Available-for-sale securities, total, weighted-average yield		4.34%
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		4
Held-to-maturity securities, Amortized Cost		5	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		4
Held-to-maturity securities, total, fair value		$ 5	7
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 3 months 18 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		7 years
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		0 years
Held-to-maturity securities, total, weighted-average maturity in years		6 years 6 months
Held-to-maturity securities, maturing in one year or less, weighted-average yield		7.47%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		6.49%
Held-to-maturity securities, maturing after five years through ten years, weighted-average yield		2.42%
Held-to-maturity securities, total, weighted-average yield		2.87%
Other Debt Securities and Obligations of Foreign Governments [Member]
Contractual Maturities of Investment Securities [Line Items]
Amortized Cost of Investment Securities		$ 17	$ 17
Available-for-sale securities, maturing in one year or less, weighted-average maturity in years		0 years
Available-for-sale securities, maturing after one year through five years, weighted-average maturity in years		0 years
Available-for-sale securities, maturing after five years through ten years, weighted-average maturity in years		0 years
Available-for-sale securities, maturing after ten years, weighted- average maturity in years		0 years
Available-for-sale securities, total, weighted-average maturity in years		0 years
Held-to-maturity securities, maturing in one year or less, amortized cost		$ 9
Held-to-maturity securities, maturing after one year through five years, amortized cost		8
Held-to-maturity securities, Amortized Cost		17
Held-to-maturity securities, maturing in one year or less, fair value		9
Held-to-maturity securities, maturing after one year through five years, fair value		8
Held-to-maturity securities, total, fair value		$ 17
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		1 year 1 month 6 days
Held-to-maturity securities, maturing after five years through ten years, weighted-average maturity in years		0 years
Held-to-maturity securities, maturing after ten years, weighted-average maturity in years		0 years
Held-to-maturity securities, total, weighted-average maturity in years		8 months 12 days
Held-to-maturity securities, maturing in one year or less, weighted-average yield		3.68%
Held-to-maturity securities, maturing after one year through five years, weighted-average yield		3.44%
Held-to-maturity securities, total, weighted-average yield		3.56%

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