Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		$ 35,856	$ 29,654
Unrealized Gains, Available-for-sale securities		448	621
Unrealized Losses, Available-for-sale securities		(367)	(135)
Fair Value, Available-for-sale securities		35,937	30,140
Amortized Cost, Held-to-Maturity Securities		14,479	13,469
Unrealized Gains, Held-to-maturity securities		297	229
Unrealized Losses, Held-to-maturity securities		(9)	(3)
Held-to-maturity Securities, Fair Value		14,767	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		12,656	11,793
Unrealized Gains, Available-for-sale securities		190	276
Unrealized Losses, Available-for-sale securities		(177)	(58)
Fair Value, Available-for-sale securities		12,669	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]	677	520
Unrealized Gains, Available-for-sale securities	[1]	3	5
Unrealized Losses, Available-for-sale securities	[1]	(4)	(1)
Fair Value, Available-for-sale securities	[1]	676	524
Amortized Cost, Held-to-Maturity Securities	[2]	4,871	4,868
Unrealized Gains, Held-to-maturity securities	[2]	146	120
Unrealized Losses, Held-to-maturity securities	[2]	(5)	(1)
Held-to-maturity Securities, Fair Value	[2]	5,012	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]	(1)	0
Fair Value, Available-for-sale securities	[1]	158	35
Direct agency obligations [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	4,145	3,995
Unrealized Gains, Available-for-sale securities	[1]	159	217
Unrealized Losses, Available-for-sale securities	[1]	(17)	(6)
Fair Value, Available-for-sale securities	[1]	4,287	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		12,785	7,985
Unrealized Gains, Available-for-sale securities		67	101
Unrealized Losses, Available-for-sale securities		(112)	(27)
Fair Value, Available-for-sale securities		12,740	8,059
Amortized Cost, Held-to-Maturity Securities		3,404	3,700
Unrealized Gains, Held-to-maturity securities		33	53
Unrealized Losses, Held-to-maturity securities		(3)	(1)
Held-to-maturity Securities, Fair Value		3,434	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		736	329
Unrealized Gains, Available-for-sale securities		12	3
Unrealized Losses, Available-for-sale securities		(1)	(2)
Fair Value, Available-for-sale securities		747	330
Amortized Cost, Held-to-Maturity Securities		6,177	4,867
Unrealized Gains, Held-to-maturity securities		116	54
Unrealized Losses, Held-to-maturity securities		(1)	(1)
Held-to-maturity Securities, Fair Value		6,292	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		763	661
Unrealized Gains, Available-for-sale securities		9	10
Unrealized Losses, Available-for-sale securities		(4)	(4)
Fair Value, Available-for-sale securities		768	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
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Held-to-Maturity Securities		8	11
Unrealized Gains, Held-to-maturity securities		1	1
Unrealized Losses, Held-to-maturity securities		0	0
Held-to-maturity Securities, Fair Value		9	12
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		22	43
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		0	0
Fair Value, Available-for-sale securities		22	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		87	97
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(8)	(8)
Fair Value, Available-for-sale securities		79	89
Other securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		111	110
Unrealized Gains, Available-for-sale securities		0	0
Unrealized Losses, Available-for-sale securities		(18)	(16)
Fair Value, Available-for-sale securities		93	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]	3,551	3,921
Unrealized Gains, Available-for-sale securities	[3]	5	6
Unrealized Losses, Available-for-sale securities	[3]	(24)	(12)
Fair Value, Available-for-sale securities	[3]	3,532	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		559	689
Equity securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		164	165
Unrealized Gains, Available-for-sale securities		3	3
Unrealized Losses, Available-for-sale securities		(1)	(1)
Fair Value, Available-for-sale securities		166	167
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		803	521
Amortized Cost, Held-to-Maturity Securities		3,289	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		$ 1,582	$ 1,683

[1] Includes securities at amortized cost of $803 million and $521 million issued or guaranteed by FNMA at September 30, 2015 and December 31, 2014, respectively, and $33 million and $34 million issued or guaranteed by the Federal Home Loan Mortgage Corporation ("FHLMC") at September 30, 2015 and December 31, 2014, respectively.

[2] Includes securities at amortized cost of $3,289 million and $3,185 million issued or guaranteed by FNMA at September 30, 2015 and December 31, 2014, respectively, and $1,582 million and $1,683 million issued and guaranteed by FHLMC at September 30, 2015 and December 31, 2014, respectively.

[3] At September 30, 2015 and December 31, 2014, foreign debt securities consisted of $559 million and $689 million, respectively, of securities fully backed by foreign governments. The remainder of foreign debt securities represents public sector entity, bank or corporate debt.