Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($)		Mar. 31, 2016		Dec. 31, 2015
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		$ 38,525,000,000		$ 35,961,000,000
Unrealized Gains, Available-for-sale securities		559,000,000		308,000,000
Unrealized Losses, Available-for-sale securities		(311,000,000)		(496,000,000)
Fair Value, Available-for-sale securities		38,773,000,000		35,773,000,000
Amortized Cost, Held-to-Maturity Securities		13,636,000,000		14,024,000,000
Unrealized Gains, Held-to-maturity securities		331,000,000		177,000,000
Unrealized Losses, Held-to-maturity securities		(1,000,000)		(36,000,000)
Held-to-maturity Securities, Fair Value		13,966,000,000		14,165,000,000
U.S. Treasury [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		18,151,000,000		17,026,000,000
Unrealized Gains, Available-for-sale securities		310,000,000		142,000,000
Unrealized Losses, Available-for-sale securities		(201,000,000)		(270,000,000)
Fair Value, Available-for-sale securities		18,260,000,000		16,898,000,000
Mortgage-backed securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	2,001,000,000		1,451,000,000
Unrealized Gains, Available-for-sale securities	[1]	22,000,000		2,000,000
Unrealized Losses, Available-for-sale securities	[1]	(1,000,000)		(12,000,000)
Fair Value, Available-for-sale securities		2,022,000,000	[1]	1,441,000,000
Amortized Cost, Held-to-Maturity Securities	[2]	2,860,000,000		2,945,000,000
Unrealized Gains, Held-to-maturity securities	[2]	64,000,000		20,000,000
Unrealized Losses, Held-to-maturity securities	[2]	0		(9,000,000)
Held-to-maturity Securities, Fair Value		2,924,000,000	[2]	2,956,000,000
Collateralized mortgage obligations [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	321,000,000		159,000,000
Unrealized Gains, Available-for-sale securities	[1]	1,000,000		0
Unrealized Losses, Available-for-sale securities	[1]	(7,000,000)		(5,000,000)
Fair Value, Available-for-sale securities		315,000,000	[1]	154,000,000
Amortized Cost, Held-to-Maturity Securities	[2]	1,675,000,000		1,755,000,000
Unrealized Gains, Held-to-maturity securities	[2]	91,000,000		73,000,000
Unrealized Losses, Held-to-maturity securities	[2]	0		(5,000,000)
Held-to-maturity Securities, Fair Value		1,766,000,000	[2]	1,823,000,000
Direct agency obligations [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[1]	4,255,000,000		4,136,000,000
Unrealized Gains, Available-for-sale securities	[1]	154,000,000		133,000,000
Unrealized Losses, Available-for-sale securities	[1]	(31,000,000)		(26,000,000)
Fair Value, Available-for-sale securities		4,378,000,000	[1]	4,243,000,000
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		11,380,000,000		10,645,000,000
Unrealized Gains, Available-for-sale securities		46,000,000		9,000,000
Unrealized Losses, Available-for-sale securities		(33,000,000)		(145,000,000)
Fair Value, Available-for-sale securities		11,393,000,000		10,509,000,000
Amortized Cost, Held-to-Maturity Securities		3,140,000,000		3,269,000,000
Unrealized Gains, Held-to-maturity securities		56,000,000		19,000,000
Unrealized Losses, Held-to-maturity securities		0		(11,000,000)
Held-to-maturity Securities, Fair Value		3,196,000,000		3,277,000,000
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		1,167,000,000		1,293,000,000
Unrealized Gains, Available-for-sale securities		25,000,000		11,000,000
Unrealized Losses, Available-for-sale securities		(2,000,000)		(4,000,000)
Fair Value, Available-for-sale securities		1,190,000,000		1,300,000,000
Amortized Cost, Held-to-Maturity Securities		5,936,000,000		6,029,000,000
Unrealized Gains, Held-to-maturity securities		118,000,000		63,000,000
Unrealized Losses, Held-to-maturity securities		(1,000,000)		(11,000,000)
Held-to-maturity Securities, Fair Value		6,053,000,000		6,081,000,000
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		25,000,000		340,000,000
Unrealized Gains, Available-for-sale securities		0		8,000,000
Unrealized Losses, Available-for-sale securities		0		0
Fair Value, Available-for-sale securities		25,000,000		348,000,000
Amortized Cost, Held-to-Maturity Securities		18,000,000		19,000,000
Unrealized Gains, Held-to-maturity securities		1,000,000		1,000,000
Unrealized Losses, Held-to-maturity securities		0		0
Held-to-maturity Securities, Fair Value		19,000,000		20,000,000
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		9,000,000		9,000,000
Unrealized Gains, Available-for-sale securities		0		0
Unrealized Losses, Available-for-sale securities		0		0
Fair Value, Available-for-sale securities		9,000,000		9,000,000
Home Equity Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		80,000,000		83,000,000
Unrealized Gains, Available-for-sale securities		0		0
Unrealized Losses, Available-for-sale securities		(10,000,000)		(8,000,000)
Fair Value, Available-for-sale securities		70,000,000		75,000,000
Other securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		515,000,000		110,000,000
Unrealized Gains, Available-for-sale securities		0		0
Unrealized Losses, Available-for-sale securities		(23,000,000)		(21,000,000)
Fair Value, Available-for-sale securities		492,000,000		89,000,000
Foreign debt securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	[3]	462,000,000		548,000,000
Unrealized Gains, Available-for-sale securities	[3]	1,000,000		0
Unrealized Losses, Available-for-sale securities	[3]	(2,000,000)		(2,000,000)
Fair Value, Available-for-sale securities		461,000,000	[3]	546,000,000
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		0
Equity securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities		159,000,000		161,000,000
Unrealized Gains, Available-for-sale securities		0		3,000,000
Unrealized Losses, Available-for-sale securities		(1,000,000)		(3,000,000)
Fair Value, Available-for-sale securities		158,000,000		161,000,000
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Held-to-Maturity Securities		7,000,000		7,000,000
Unrealized Gains, Held-to-maturity securities		1,000,000		1,000,000
Unrealized Losses, Held-to-maturity securities		0		0
Held-to-maturity Securities, Fair Value		8,000,000		8,000,000
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		2,277,000,000		1,577,000,000
Amortized Cost, Held-to-Maturity Securities		3,074,000,000		3,182,000,000
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		45,000,000		33,000,000
Amortized Cost, Held-to-Maturity Securities		$ 1,461,000,000		$ 1,518,000,000

[1] Includes securities at amortized cost of $2,277 million and $1,577 million issued or guaranteed by FNMA at March 31, 2016 and December 31, 2015, respectively, and $45 million and $33 million issued or guaranteed by the Federal Home Loan Mortgage Corporation ("FHLMC") at March 31, 2016 and December 31, 2015, respectively.

[2] Includes securities at amortized cost of $3,074 million and $3,182 million issued or guaranteed by FNMA at March 31, 2016 and December 31, 2015, respectively, and $1,461 million and $1,518 million issued and guaranteed by FHLMC at March 31, 2016 and December 31, 2015, respectively.

[3] At both March 31, 2016 and December 31, 2015, none of our foreign debt securities were fully backed by foreign governments. Foreign debt securities represent public sector entity, bank or corporate debt.