Financial Guaranty Insurance Contracts (Details 14) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2012 contract	Dec. 31, 2011 contract	Mar. 31, 2011	Dec. 31, 2010
Reserves (salvage)
Total BIG, Net	$ 248,100,000	$ 86,100,000
Effect of consolidating FG VIEs	229,100,000	196,500,000
Total	477,200,000	282,600,000
Interest rate swaps notional exposure	17,200,000,000
Maximum potential liability under interest rate swaps as a result of downgrade of the financial strength ratings	800,000,000
Number of the entity's insurance subsidiaries that results in increased claims due to downgrade		1
Minimum number of rating agency considered for triggering bank termination right		1
Minimum
Reserves (salvage)
Threshold period of bonds held by bank for right of accelerated repayment (in days)	180 days
Accelerated repayment period for bank bonds (in years)	10 years
Variable rate demand obligations
Reserves (salvage)
Rate basis for bank bond rate	prime rate
Par of insured variable rate demand obligations issued by municipal obligors with internal ratings of BBB- or lower	1,100,000,000
Variable rate demand obligations | Minimum
Reserves (salvage)
Bank bond rate (as a percent)	2.00%
Threshold period of bonds held by bank for right of accelerated repayment (in days)	90 days
Accelerated repayment period for bank bonds (in years)	5 years
Variable rate demand obligations | Maximum
Reserves (salvage)
Bank bond rate (as a percent)	3.00%
Bank bond capped rate (as a percent)	25.00%
Financial Guaranty
Number of risks
Total BIG, Net (in contracts)	368	368
Total (in contracts)	368	368
Remaining weighted average contract period
Total BIG, Net (in years)	10 years 7 months 6 days	10 years 4 months 24 days
Total (in years)	10 years 7 months 6 days	10 years 4 months 24 days
Principal
Total BIG, Net	18,519,300,000	18,958,500,000
Total	18,519,300,000	18,958,500,000
Interest
Total BIG, Net	8,607,200,000	8,623,300,000
Total	8,607,200,000	8,623,300,000
Total net outstanding exposure
Total BIG, Net	27,126,500,000	27,581,800,000
Total	27,126,500,000	27,581,800,000
Expected cash outflows (inflows)
Total BIG, Net	5,459,900,000	5,074,400,000
Effect of Consolidating FG VIEs	(818,200,000)	(998,400,000)
Total	4,641,700,000	4,076,000,000
Potential recoveries
Total BIG, Net	(4,346,800,000)	(4,115,000,000)
Effect of Consolidating FG VIEs	905,600,000	1,059,800,000
Total	(3,441,200,000)	(3,055,200,000)
Subtotal
Total BIG, NET	1,113,100,000	959,400,000
Effect of Consolidating FG VIEs	87,400,000	61,400,000
Total	1,200,500,000	1,020,800,000
Discount
Total BIG, Net	(360,700,000)	(321,400,000)
Effect of Consolidating FG VIEs	68,100,000	45,300,000
Total	(292,600,000)	(276,100,000)
Present value of expected cash flows
Total BIG, Net	752,400,000	638,000,000	32,200,000	408,800,000
Effect of Consolidating FG VIEs	155,500,000	106,700,000
Total	907,900,000	744,700,000
Deferred premium revenue
Total BIG, Net	1,169,700,000	1,325,500,000
Effect of Consolidating FG VIEs	(343,800,000)	(390,700,000)
Total	825,900,000	934,800,000
Reserves (salvage)
Total BIG, Net	246,200,000	84,200,000
Effect of consolidating FG VIEs	229,100,000	196,500,000
Total	475,300,000	280,700,000
Financial Guaranty | BIG 1
Number of risks
Gross (in contracts)	164	171
Ceded (in contracts)	(58)	(68)
Remaining weighted average contract period
Gross (in years)	10 years 3 months 18 days	10 years
Ceded (in years)	9 years 1 month 6 days	9 years 2 months 12 days
Principal
Gross	9,048,800,000	9,675,800,000
Ceded	(1,345,600,000)	(1,378,000,000)
Interest
Gross	4,143,500,000	4,307,900,000
Ceded	(464,500,000)	(485,600,000)
Total net outstanding exposure
Gross	13,192,300,000	13,983,700,000
Ceded	(1,810,100,000)	(1,863,600,000)
Expected cash outflows (inflows)
Gross	1,567,600,000	1,730,600,000
Ceded	(646,000,000)	(658,800,000)
Potential recoveries
Gross	(1,688,000,000)	(1,798,000,000)
Ceded	669,800,000	664,000,000
Subtotal
Gross	(120,400,000)	(67,400,000)
Ceded	23,800,000	5,200,000
Discount
Gross	9,900,000	15,700,000
Ceded	(4,800,000)	(4,600,000)
Present value of expected cash flows
Gross	(110,500,000)	(51,700,000)
Ceded	19,000,000	600,000
Deferred premium revenue
Gross	112,300,000	260,800,000
Ceded	(13,600,000)	(69,100,000)
Reserves (salvage)
Gross	(136,600,000)	(96,600,000)
Ceded	24,100,000	6,900,000
Financial Guaranty | BIG 2
Number of risks
Gross (in contracts)	79	71
Ceded (in contracts)	(27)	(26)
Remaining weighted average contract period
Gross (in years)	13 years 3 months 18 days	13 years 8 months 12 days
Ceded (in years)	25 years 6 months	20 years 6 months
Principal
Gross	4,192,300,000	3,731,600,000
Ceded	(289,500,000)	(274,000,000)
Interest
Gross	3,206,100,000	2,889,400,000
Ceded	(515,400,000)	(404,800,000)
Total net outstanding exposure
Gross	7,398,400,000	6,621,000,000
Ceded	(804,900,000)	(678,800,000)
Expected cash outflows (inflows)
Gross	2,203,400,000	1,833,300,000
Ceded	(233,300,000)	(120,300,000)
Potential recoveries
Gross	(1,049,200,000)	(1,079,300,000)
Ceded	50,000,000	38,500,000
Subtotal
Gross	1,154,200,000	754,000,000
Ceded	(183,300,000)	(81,800,000)
Discount
Gross	(279,100,000)	(240,600,000)
Ceded	23,100,000	31,600,000
Present value of expected cash flows
Gross	875,100,000	513,400,000
Ceded	(160,200,000)	(50,200,000)
Deferred premium revenue
Gross	311,500,000	280,900,000
Ceded	(32,000,000)	(12,300,000)
Reserves (salvage)
Gross	637,000,000	319,500,000
Ceded	(141,700,000)	(41,900,000)
Financial Guaranty | BIG 3
Number of risks
Gross (in contracts)	125	126
Ceded (in contracts)	(48)	(48)
Remaining weighted average contract period
Gross (in years)	9 years 3 months 18 days	9 years 2 months 12 days
Ceded (in years)	6 years 6 months	6 years 4 months 24 days
Principal
Gross	7,512,500,000	7,830,800,000
Ceded	(599,200,000)	(627,700,000)
Interest
Gross	2,401,300,000	2,486,400,000
Ceded	(163,800,000)	(170,000,000)
Total net outstanding exposure
Gross	9,913,800,000	10,317,200,000
Ceded	(763,000,000)	(797,700,000)
Expected cash outflows (inflows)
Gross	2,691,800,000	2,423,000,000
Ceded	(123,600,000)	(133,400,000)
Potential recoveries
Gross	(2,423,400,000)	(2,040,500,000)
Ceded	94,000,000	100,300,000
Subtotal
Gross	268,400,000	382,500,000
Ceded	(29,600,000)	(33,100,000)
Discount
Gross	(110,500,000)	(125,100,000)
Ceded	700,000	1,600,000
Present value of expected cash flows
Gross	157,900,000	257,400,000
Ceded	(28,900,000)	(31,500,000)
Deferred premium revenue
Gross	899,000,000	991,800,000
Ceded	(107,500,000)	(126,600,000)
Reserves (salvage)
Gross	(145,500,000)	(110,200,000)
Ceded	$ 8,900,000	$ 6,500,000