Financial Guaranty Contracts Accounted for as Credit Derivatives (Details 5) (USD $)	12 Months Ended
	Dec. 31, 2011 Counterparty Agreement	Dec. 31, 2010	Dec. 31, 2011 AGC	Dec. 31, 2010 AGC	Dec. 31, 2009 AGC	Dec. 31, 2008 AGC	Dec. 31, 2007 AGC	Dec. 31, 2011 AGM	Dec. 31, 2010 AGM	Dec. 31, 2009 AGM	Jul. 01, 2009 AGM
Credit Derivatives
Quoted price of CDS contract (as a percent)			11.40%	8.04%	6.34%	17.75%	1.90%	7.78%	6.50%	5.41%	10.47%
Components of Credit Derivative Assets (Liabilities)
Credit derivative assets	$ 468,933,000	$ 592,898,000
Credit derivative liabilities	(1,772,803,000)	(2,462,831,000)
Net fair value of credit derivatives	(1,303,900,000)	(1,869,900,000)
Less: Effect of AGC and AGM credit spreads	4,291,900,000	3,669,400,000
Fair value of credit derivatives before effect of AGC and AGM credit spreads	(5,595,800,000)	(5,539,300,000)
Other credit derivative disclosures
Loss from widening of spreads	56,500,000
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	2,300,000,000
Number of counterparties that could terminate for ratings downgrade to BB+ or Ba1	2
Amount of par subject to collateral posting	14,800,000,000
Collateral agreed to be posted	779,900,000
Amount of par subject to collateral for which the amount of collateral is capped	14,300,000,000
Cap on collateral requirement at current ratings levels	625,000,000
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	$ 675,000,000