Securities Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) (USD $)	Mar. 31, 2013		Dec. 31, 2012
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	$ 58,040,000,000		$ 66,041,000,000
Unrealized Gains, Available-for-sale securities	1,572,000,000		1,828,000,000
Unrealized Losses, Available-for-sale securities	(154,000,000)		(153,000,000)
Fair Value, Available-for-sale securities	59,458,000,000		67,716,000,000
Amortized Cost, Held-to-Maturity Securities	1,434,000,000		1,620,000,000
Unrealized Gains, Held-to-Maturity Securities	200,000,000		211,000,000
Unrealized Losses, Held-to-Maturity Securities	0		0
Fair Value, Held-to-Maturity Securities	1,634,000,000		1,831,000,000
Held-to-maturity Securities, Fair Value	1,600,000,000		1,800,000,000
U.S. Treasury [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	26,715,000,000		34,800,000,000
Unrealized Gains, Available-for-sale securities	475,000,000		566,000,000
Unrealized Losses, Available-for-sale securities	(26,000,000)		(24,000,000)
Fair Value, Available-for-sale securities	27,164,000,000		35,342,000,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	176,000,000	[1]	166,000,000	[1]
Unrealized Gains, Available-for-sale securities	1,000,000	[1]	1,000,000	[1]
Unrealized Losses, Available-for-sale securities	(2,000,000)	[1]	(1,000,000)	[1]
Fair Value, Available-for-sale securities	175,000,000	[1]	166,000,000	[1]
Amortized Cost, Held-to-Maturity Securities	1,035,000,000	[2]	1,121,000,000	[2]
Unrealized Gains, Held-to-Maturity Securities	146,000,000	[2]	148,000,000	[2]
Unrealized Losses, Held-to-Maturity Securities	0	[2]	0	[2]
Fair Value, Held-to-Maturity Securities	1,181,000,000	[2]	1,269,000,000	[2]
Direct Agency Obligations U S Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	4,029,000,000	[1]	4,039,000,000	[1]
Unrealized Gains, Available-for-sale securities	374,000,000	[1]	364,000,000	[1]
Unrealized Losses, Available-for-sale securities	(2,000,000)	[1]	(2,000,000)	[1]
Fair Value, Available-for-sale securities	4,401,000,000	[1]	4,401,000,000	[1]
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	15,471,000,000		15,646,000,000
Unrealized Gains, Available-for-sale securities	536,000,000		674,000,000
Unrealized Losses, Available-for-sale securities	(8,000,000)		(6,000,000)
Fair Value, Available-for-sale securities	15,999,000,000		16,314,000,000
Amortized Cost, Held-to-Maturity Securities	62,000,000		66,000,000
Unrealized Gains, Held-to-Maturity Securities	11,000,000		12,000,000
Unrealized Losses, Held-to-Maturity Securities	0		0
Fair Value, Held-to-Maturity Securities	73,000,000		78,000,000
Collateralized Mortgage Obligations Issued By Private Enterprises [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	4,865,000,000		4,315,000,000
Unrealized Gains, Available-for-sale securities	130,000,000		156,000,000
Unrealized Losses, Available-for-sale securities	(19,000,000)		0
Fair Value, Available-for-sale securities	4,976,000,000		4,471,000,000
Amortized Cost, Held-to-Maturity Securities	261,000,000		277,000,000
Unrealized Gains, Held-to-Maturity Securities	39,000,000		42,000,000
Unrealized Losses, Held-to-Maturity Securities	0		0
Fair Value, Held-to-Maturity Securities	300,000,000		319,000,000
Direct Agency Obligations Issued By Private Enterprises [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	1,000,000		1,000,000
Unrealized Gains, Available-for-sale securities	0		0
Unrealized Losses, Available-for-sale securities	0		0
Fair Value, Available-for-sale securities	1,000,000		1,000,000
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	743,000,000		877,000,000
Unrealized Gains, Available-for-sale securities	30,000,000		37,000,000
Unrealized Losses, Available-for-sale securities	(3,000,000)		(2,000,000)
Fair Value, Available-for-sale securities	770,000,000		912,000,000
Amortized Cost, Held-to-Maturity Securities	37,000,000		38,000,000
Unrealized Gains, Held-to-Maturity Securities	2,000,000		3,000,000
Unrealized Losses, Held-to-Maturity Securities	0		0
Fair Value, Held-to-Maturity Securities	39,000,000		41,000,000
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	1,000,000		1,000,000
Unrealized Gains, Available-for-sale securities	0		0
Unrealized Losses, Available-for-sale securities	0		0
Fair Value, Available-for-sale securities	1,000,000		1,000,000
Amortized Cost, Held-to-Maturity Securities	39,000,000		118,000,000
Unrealized Gains, Held-to-Maturity Securities	2,000,000		6,000,000
Unrealized Losses, Held-to-Maturity Securities	0		0
Fair Value, Held-to-Maturity Securities	41,000,000		124,000,000
Commercial Mortgages Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	180,000,000		208,000,000
Unrealized Gains, Available-for-sale securities	5,000,000		6,000,000
Unrealized Losses, Available-for-sale securities	0		0
Fair Value, Available-for-sale securities	185,000,000		214,000,000
Home Equity Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	299,000,000		310,000,000
Unrealized Gains, Available-for-sale securities	0		0
Unrealized Losses, Available-for-sale securities	(44,000,000)		(52,000,000)
Fair Value, Available-for-sale securities	255,000,000		258,000,000
Student Loans Backed Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	0		0
Unrealized Gains, Available-for-sale securities	0		0
Unrealized Losses, Available-for-sale securities	0		0
Fair Value, Available-for-sale securities	0		0
Other Debt Obligations [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	102,000,000		102,000,000
Unrealized Gains, Available-for-sale securities	0		0
Unrealized Losses, Available-for-sale securities	(17,000,000)		(18,000,000)
Fair Value, Available-for-sale securities	85,000,000		84,000,000
Corporate and other domestic debt securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	24,000,000		24,000,000
Unrealized Gains, Available-for-sale securities	2,000,000		2,000,000
Unrealized Losses, Available-for-sale securities	0		0
Fair Value, Available-for-sale securities	26,000,000		26,000,000
Foreign debt securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	5,267,000,000	[3],[4]	5,385,000,000	[3],[4]
Unrealized Gains, Available-for-sale securities	14,000,000	[3],[4]	16,000,000	[3],[4]
Unrealized Losses, Available-for-sale securities	(33,000,000)	[3],[4]	(48,000,000)	[3],[4]
Fair Value, Available-for-sale securities	5,248,000,000	[3],[4]	5,353,000,000	[3],[4]
Equity securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	167,000,000		167,000,000
Unrealized Gains, Available-for-sale securities	5,000,000		6,000,000
Unrealized Losses, Available-for-sale securities	0		0
Fair Value, Available-for-sale securities	172,000,000		173,000,000
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Held-to-maturity Securities, Fair Value	477,000,000		507,000,000
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] | US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	152,000,000		153,000,000
Foreign Government [Member] | Foreign debt securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Foreign Debt Securities Fully Backed By Foreign Governments	1,400,000,000		1,500,000,000
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Held-to-maturity Securities, Fair Value	558,000,000		614,000,000
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member] | US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities and Held-to-Maturity Securities [Line Items]
Amortized Cost, Available-for-sale securities	$ 24,000,000		$ 13,000,000

[1] Includes securities at amortized cost of $152 million and $153 million issued or guaranteed by FNMA at March 31, 2013 and December 31, 2012, respectively, and $24 million and $13 million issued or guaranteed by FHLMC at March 31, 2013 and December 31, 2012, respectively.

[2] Includes securities at amortized cost of $477 million and $507 million issued or guaranteed by FNMA at March 31, 2013 and December 31, 2012, respectively, and $558 million and $614 million issued and guaranteed by FHLMC at March 31, 2013 and December 31, 2012, respectively.

[3] At March 31, 2013 and December 31, 2012, foreign debt securities consisted of $1.4 billion and $1.5 billion, respectively, of securities fully backed by foreign governments. The remainder of foreign debt securities represents foreign bank or corporate debt.

[4] We did not hold any foreign debt securities issued by the governments of Greece, Ireland, Italy, Portugal or Spain at March 31, 2013 and December 31, 2012.