Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 5) (USD $)	9 Months Ended
	Sep. 30, 2011 subsidiary statement ratingcategory security Y segment entity agreement market company policy D transaction counterparty	Dec. 31, 2010	Sep. 30, 2011 AGC	Jun. 30, 2011 AGC	Dec. 31, 2010 AGC	Jun. 30, 2010 AGC	Dec. 31, 2009 AGC	Sep. 30, 2011 AGM	Jun. 30, 2011 AGM	Dec. 31, 2010 AGM	Jun. 30, 2010 AGM	Dec. 31, 2009 AGM
Credit Derivatives
Quoted price of CDS contract (in basis points)			13.51%	6.34%	8.04%	10.10%	6.34%	10.04%	4.72%	6.50%	8.02%	5.41%
Components of Credit Derivative Assets (Liabilities)
Credit derivative assets	$ 467,252,000	$ 592,898,000
Credit derivative liabilities	(1,495,312,000)	(2,462,831,000)
Net fair value of credit derivatives	(1,028,100,000)	(1,869,900,000)
Less: Effect of AGC and AGM credit spreads	4,938,600,000	3,669,400,000
Fair value of credit derivatives before effect of AGC and AGM credit spreads	(5,966,700,000)	(5,539,300,000)
Other credit derivative disclosures
Loss from tightening of general market spreads as well as a run-off in net par outstanding	439,400,000
Par insured that could be terminated if ratings were downgraded	2,400,000,000
Number of ISDA master agreements	3
Par insured that could be terminated if ratings were downgraded to BBB- or Baa3	89,000,000
Number of counterparties that could terminate for ratings downgrade to BBB- or Baa3	1
Par insured that could be terminated if ratings were downgraded to BB+ or Ba1	2,400,000,000
Number of counterparties that could terminate for ratings downgrade to BB+ or Ba1	2
Amount of par subject to collateral posting	15,400,000,000
Collateral agreed to be posted	774,300,000
Amount of par subject to collateral for which the amount of collateral is capped	14,800,000,000
Cap on collateral requirement at current ratings levels	635,000,000
Increase to cap on collateral requirement if ratings are downgraded to A+ or A3	50,000,000
Cap on collateral requirement if ratings are downgraded to A+ or A3	$ 685,000,000