Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		$ 35,961	$ 29,654
Unrealized Gains, Available-for-sale securities		308	621
Unrealized Losses, Available-for-sale securities		(496)	(135)
Fair Value, Available-for-sale securities		35,773	30,140
Amortized Cost, Held-to-Maturity Securities		14,024	13,469
Unrealized Gains, Held-to-maturity securities		177	229
Unrealized Losses, Held-to-maturity securities		(36)	(3)
Held-to-maturity Securities, Fair Value		14,165	13,695
U.S. Treasury [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		17,026	11,793
Unrealized Gains, Available-for-sale securities		142	276
Unrealized Losses, Available-for-sale securities		(270)	(58)
Fair Value, Available-for-sale securities		16,898	12,011
Mortgage-backed securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	1,451	520
Unrealized Gains, Available-for-sale securities	[1]	2	5
Unrealized Losses, Available-for-sale securities	[1]	(12)	(1)
Fair Value, Available-for-sale securities	[1]	1,441	524
Amortized Cost, Held-to-Maturity Securities	[2]	2,945	4,868
Unrealized Gains, Held-to-maturity securities	[2]	20	120
Unrealized Losses, Held-to-maturity securities	[2]	(9)	(1)
Held-to-maturity Securities, Fair Value	[2]	2,956	4,987
Collateralized mortgage obligations [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	159	35
Unrealized Gains, Available-for-sale securities	[1]	0	0
Unrealized Losses, Available-for-sale securities	[1]	(5)	0
Fair Value, Available-for-sale securities	[1]	154	35
Amortized Cost, Held-to-Maturity Securities	[2]	1,755
Unrealized Gains, Held-to-maturity securities	[2]	73
Unrealized Losses, Held-to-maturity securities	[2]	(5)
Held-to-maturity Securities, Fair Value	[2]	1,823
Direct agency obligations [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	4,136	3,995
Unrealized Gains, Available-for-sale securities	[1]	133	217
Unrealized Losses, Available-for-sale securities	[1]	(26)	(6)
Fair Value, Available-for-sale securities	[1]	4,243	4,206
Mortgage-backed securities, issued or guaranteed by US Government agency [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		10,645	7,985
Unrealized Gains, Available-for-sale securities		9	101
Unrealized Losses, Available-for-sale securities		(145)	(27)
Fair Value, Available-for-sale securities		10,509	8,059
Amortized Cost, Held-to-Maturity Securities		3,269	3,700
Unrealized Gains, Held-to-maturity securities		19	53
Unrealized Losses, Held-to-maturity securities		(11)	(1)
Held-to-maturity Securities, Fair Value		3,277	3,752
Collateralized mortgage obligations, issued or guaranteed by US Government agency [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		1,293	329
Unrealized Gains, Available-for-sale securities		11	3
Unrealized Losses, Available-for-sale securities		(4)	(2)
Fair Value, Available-for-sale securities		1,300	330
Amortized Cost, Held-to-Maturity Securities		6,029	4,867
Unrealized Gains, Held-to-maturity securities		63	54
Unrealized Losses, Held-to-maturity securities		(11)	(1)
Held-to-maturity Securities, Fair Value		6,081	4,920
Obligations of U.S. states and political subdivision [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		340	661
Unrealized Gains, Available-for-sale securities		8	10
Unrealized Losses, Available-for-sale securities		0	(4)
Fair Value, Available-for-sale securities		348	667
Amortized Cost, Held-to-Maturity Securities		19	23
Unrealized Gains, Held-to-maturity securities		1	1
Unrealized Losses, Held-to-maturity securities		0	0
Held-to-maturity Securities, Fair Value		20	24
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		9	43
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		0	0
Fair Value, Available-for-sale securities		9	43
Home Equity Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		83	97
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(8)	(8)
Fair Value, Available-for-sale securities		75	89
Other securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		110	110
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(21)	(16)
Fair Value, Available-for-sale securities		89	94
Foreign Debt Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[3]	548	3,921
Unrealized Gains, Available-for-sale securities	[3]	0	6
Unrealized Losses, Available-for-sale securities	[3]	(2)	(12)
Fair Value, Available-for-sale securities	[3]	546	3,915
Foreign Debt Securities [Member] | Foreign Government [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		0	689
Equity securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		161	165
Unrealized Gains, Available-for-sale securities		3	3
Unrealized Losses, Available-for-sale securities		(3)	(1)
Fair Value, Available-for-sale securities		161	167
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Held-to-Maturity Securities		7	11
Unrealized Gains, Held-to-maturity securities		1	1
Unrealized Losses, Held-to-maturity securities		0	0
Held-to-maturity Securities, Fair Value		8	12
US Government sponsored enterprises [Member] | Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		1,577	521
Amortized Cost, Held-to-Maturity Securities		3,182	3,185
US Government sponsored enterprises [Member] | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		33	34
Amortized Cost, Held-to-Maturity Securities		$ 1,518	$ 1,683

[1] Includes securities at amortized cost of $1,577 million and $521 million issued or guaranteed by FNMA at December 31, 2015 and 2014, respectively, and $33 million and $34 million issued or guaranteed by the Federal Home Loan Mortgage Corporation ("FHLMC") at December 31, 2015 and 2014, respectively.

[2] Includes securities at amortized cost of $3,182 million and $3,185 million issued or guaranteed by FNMA at December 31, 2015 and 2014, respectively, and $1,518 million and $1,683 million issued and guaranteed by FHLMC at December 31, 2015 and 2014, respectively.

[3] At December 31, 2015 none of our foreign debt securities were fully backed by foreign governments. At December 31, 2014, foreign debt securities consisted of $689 million of securities fully backed by foreign governments. The remainder of foreign debt securities represents public sector entity, bank or corporate debt.