Financial Guaranty Insurance Contracts (Details 10)					12 Months Ended			1 Months Ended	12 Months Ended										12 Months Ended					12 Months Ended			12 Months Ended
	Dec. 31, 2011 USD ($) Transaction	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2011 Maximum USD ($)	Dec. 31, 2011 "XXX" Life Insurance Transactions USD ($)	Dec. 31, 2011 "XXX" Life Insurance Transactions Below investment grade ("BIG") USD ($)	Dec. 31, 2011 Student Loan USD ($)	Jun. 30, 2011 Trust preferred securities collateralized debt obligations Transaction	Dec. 31, 2011 Trust preferred securities collateralized debt obligations USD ($)	Dec. 31, 2011 Trust preferred securities collateralized debt obligations Below investment grade ("BIG") USD ($)	Dec. 31, 2011 Public finance student loan USD ($)	Dec. 31, 2011 Public finance student loan Below investment grade ("BIG") USD ($)	Dec. 31, 2011 Asset-backed student loan USD ($)	Dec. 31, 2011 Asset-backed student loan Below investment grade ("BIG") USD ($)	Dec. 31, 2011 Jefferson County Alabama Sewer Authority USD ($)	Dec. 31, 2010 Jefferson County Alabama Sewer Authority USD ($)	Dec. 31, 2011 Telephone directory "yellow pages" USD ($)	Dec. 31, 2010 Telephone directory "yellow pages" USD ($)	Dec. 31, 2011 Manufactured housing loans USD ($) Transaction	Dec. 31, 2010 Manufactured housing loans USD ($)	Dec. 31, 2011 Manufactured housing loans Below investment grade ("BIG") USD ($)	Dec. 31, 2011 Harrisburg, Pennsylvania USD ($)	Dec. 31, 2011 Harrisburg, Pennsylvania Below investment grade ("BIG") USD ($)	Dec. 31, 2011 Sovereign debt of Greece with a 4.5% fixed coupon EUR (€)	Dec. 31, 2011 Jefferson County Alabama Sewer Authority Warrants USD ($)	Dec. 31, 2011 Jefferson County Alabama Sewer Authority Bonds USD ($)	Dec. 31, 2011 Sovereign debt of Greece due in 2057 with a 2.085% inflation-linked coupon EUR (€)
Financial guarantee obligations
Net exposure of debt																								€ 165,100,000			€ 52,600,000
Exposure of debt																								200,000,000			113,900,000
Coupon rate (as a percent)																								4.50%			2.085%
Reduction of notional amount due to relief measures related to sovereign debt of Greece (as a percent)																								53.50%			53.50%
Weighted-average discounted loss posted by the Company, gross of reinsurance				64,700,000
Weighted average discounted loss posted by the Company, net of reinsurance				42,600,000
Net Par Outstanding Financial Guaranty Insurance					2,300,000,000	882,500,000	3,000,000,000		1,800,000,000	940,600,000	1,500,000,000	237,600,000	1,500,000,000	614,400,000	731,800,000		110,700,000		339,600,000		228,600,000	168,600,000	95,300,000		495,800,000	236,000,000
Gross expected loss	679,011,000	574,369,000	306,400,000		129,500,000		74,600,000								26,700,000	33,000,000	21,900,000	1,100,000
Largest expected loss to be paid for a single transaction							27,500,000		13,200,000																0	1,000,000
Projected net expected loss							3,300,000								6,300,000		20,800,000
Amount of transactions insured converted from CDS form to financial guaranty form									835,100,000
Number of transactions for which estimated loss was projected																			2
Number of TruPS CDOs converted to financial guaranty form								2
Number of direct transactions for which estimated loss was projected																			2
Expected loss on direct transactions																			18,400,000	14,300,000					218,700,000	170,900,000
Net par of contracts for which estimated loss was projected																			144,800,000
Expected loss on assumed transactions																			5,400,000						277,100,000	65,100,000
Number of assumed transactions for which estimated loss was projected																			11
Net par of assumed contracts for which estimated loss was projected																			83,700,000
Net claims to date																						6,900,000
Number of U.S. RMBS transactions for which lawsuits were filed alleging breaches of R&W	6
Number of U.S. RMBS transactions for which lawsuits were filed alleging violations of state securities laws and other claims	2
Increase in projected expected loss to be paid					$ 55,700,000				$ 13,800,000