Investments (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended								9 Months Ended				9 Months Ended				9 Months Ended						9 Months Ended	12 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Sep. 30, 2011 Asset-Backed Securities [Member] Asset / Liability Products [Member]	Sep. 30, 2011 Asset / Liability Products [Member]	Sep. 30, 2010 Asset / Liability Products [Member]	Sep. 30, 2011 Asset / Liability Products [Member]	Sep. 30, 2010 Asset / Liability Products [Member]	Sep. 30, 2011 National [Member] Corporate Obligations [Member]	Sep. 30, 2011 Mortgage-Backed Securities [Member] 3rd Party Guarantors [Member]	Sep. 30, 2011 Asset-Backed Securities [Member] 3rd Party Guarantors [Member]	Sep. 30, 2011 3rd Party Guarantors [Member] Corporate Obligations [Member]	Sep. 30, 2011 Book Value Greater Than Market Value By More Than 5% [Member]	Dec. 31, 2010 Book Value Greater Than Market Value By More Than 5% [Member]	Sep. 30, 2011 Asset-Backed Securities [Member]	Sep. 30, 2011 Asset-Backed Securities [Member] MBIA [Member]		Sep. 30, 2011 Asset-Backed Securities [Member] MBIA Corp [Member]	Sep. 30, 2011 Mortgage-Backed Securities [Member]	Sep. 30, 2011 Mortgage-Backed Securities [Member] MBIA [Member]		Sep. 30, 2011 Mortgage-Backed Securities [Member] MBIA Corp [Member]	Sep. 30, 2011 Corporate Obligations [Member]	Sep. 30, 2011 Corporate Obligations [Member] MBIA [Member]		Sep. 30, 2011 Corporate Obligations [Member] MBIA Corp [Member]	Sep. 30, 2011 Variable Interest Entity, Primary Beneficiary [Member]	Dec. 31, 2010 Variable Interest Entity, Primary Beneficiary [Member]	Sep. 30, 2011 Aggregate Unrealized Losses [Member]	Dec. 31, 2010 Aggregate Unrealized Losses [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of securities on deposit with various regulatory authorities	$ 11,000,000		$ 11,000,000		$ 11,000,000
Fair value of securities pledged as collateral	1,800,000,000		1,800,000,000		2,400,000,000
Cash pledged as collateral under investment agreements	378,000,000		378,000,000		113,000,000
Unrecognized gross losses																													394,000,000	279,000,000
Gross unrealized losses, portfolio	961,000,000		961,000,000		1,100,000,000
Gross unrealized losses, available for sale securities	(567,000,000)		(567,000,000)		(804,000,000)
12 months or longer, unrealized losses																															893,000,000	941,000,000
Rate of book value securities exceeded market value	5.00%		5.00%		5.00%
Number of securities, unrealized loss position for continuous 12-month period or longer	313		313		412										215	321
Weighted average contractual maturity period of securities, unrealized loss position	22		22		20
Rate of security impaired at time of review			20.00%
Rate of security impaired at time of review, fair value below amortized cost for consecutive 12-month period			5.00%
Percentage of investments rated investment grade																	60.00%				62.00%
Investment grade rated with AAA																	21.00%				21.00%				9.00%
Fair value	2,549,000,000		2,549,000,000		6,954,000,000	413,000,000											723,000,000	323,000,000	[1]		450,000,000	18,000,000	[1]		774,000,000	135,000,000	[1]
Gross unrealized losses	(567,000,000)		(567,000,000)		(804,000,000)	124,000,000											(285,000,000)	80,000,000	[1]		121,000,000	5,000,000	[1]		109,000,000	19,000,000	[1]
Other-than-temporary impairments	11,000,000	0	44,000,000	43,000,000			1,000,000	0	30,000,000	39,000,000
Benefit of Guarantees Provided											21,000,000	259,000,000	119,000,000	54,000,000						323,000,000				18,000,000				114,000,000
Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors																	$ 239,000,000				$ 257,000,000				$ 132,000,000
Securities rated below investment grade percentage without giving effect to the benefit of guarantees provided by financial guarantors																	33.00%				57.00%				17.00%

[1] Includes investments insured by MBIA Corp. and National.