Financial Guaranty Insurance Contracts (Details 12) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2011 Y contract	Dec. 31, 2010 Y contract	Jun. 30, 2011	Sep. 30, 2010	Jun. 30, 2010	Dec. 31, 2009
Reserves (salvage)
Total BIG, Net	$ 146.1	$ (393.6)
Effect of consolidating FG VIEs	162.0	42.7
Total	308.1	(350.9)
Financial Guaranty
Number of risks
Total BIG, Net	364	332
Total	364	332
Remaining weighted average contract period
Total BIG, Net (in years)	10.1	9.6
Total (in years)	10.1	9.6
Principal
Total BIG, Net	19,074.2	18,447.1
Total	19,074.2	18,447.1
Interest
Total BIG, Net	8,546.9	7,854.2
Total	8,546.9	7,854.2
Total net outstanding exposure
Total BIG, Net	27,621.1	26,301.3
Total	27,621.1	26,301.3
Expected cash flows
Total BIG, Net	4,100.4	4,374.8
Effect of Consolidating VIEs	(591.2)	(384.2)
Total	3,509.2	3,990.6
Less: Potential recoveries
Total BIG, Net	(2,947.0)	(3,200.3)
Effect of Consolidating VIEs	599.2	354.8
Total	(2,347.8)	(2,845.5)
Subtotal
Total BIG, NET	1,153.4	1,174.5
Effect of Consolidating VIEs	8.0	(29.4)
Total	1,161.4	1,145.1
Discount
Total BIG, Net	(380.0)	(765.7)
Effect of Consolidating VIEs	26.2	(19.8)
Total	(353.8)	(785.5)
Present value of expected cash flows
Total BIG, Net	773.4	408.8	713.0	589.3	766.9	1,066.4
Effect of Consolidating VIEs	34.2	(49.2)		30.5
Total	807.6	359.6		619.8
Deferred premium revenue
Total BIG, Net	1,322.4	1,570.3
Effect of Consolidating VIEs	(405.9)	(263.9)
Total	916.5	1,306.4
Reserves (salvage)
Total BIG, Net	144.2	(395.7)
Effect of consolidating FG VIEs	162.0	42.7
Total	306.2	(353.0)
BIG 1 | Financial Guaranty
Number of risks
Gross	165	119
Ceded	(62)	(45)
Remaining weighted average contract period
Gross (in years)	11.9	11.7
Ceded (in years)	14.6	16.0
Principal
Gross	7,451.1	6,173.0
Ceded	(760.5)	(723.3)
Interest
Gross	4,269.4	3,599.5
Ceded	(561.3)	(580.4)
Total net outstanding exposure
Gross	11,720.5	9,772.5
Ceded	(1,321.8)	(1,303.7)
Expected cash flows
Gross	456.7	303.9
Ceded	(89.1)	(20.2)
Less: Potential recoveries
Gross	(616.3)	(375.2)
Ceded	114.0	37.4
Subtotal
Gross	(159.6)	(71.3)
Ceded	24.9	17.2
Discount
Gross	47.2	(21.0)
Ceded	(2.9)	(5.5)
Present value of expected cash flows
Gross	(112.4)	(92.3)
Ceded	22.0	11.7
Deferred premium revenue
Gross	96.8	169.9
Ceded	(18.3)	(16.9)
Reserves (salvage)
Gross	(128.0)	(112.9)
Ceded	25.5	12.4
BIG 2 | Financial Guaranty
Number of risks
Gross	72	98
Ceded	(26)	(42)
Remaining weighted average contract period
Gross (in years)	9.7	8.4
Ceded (in years)	6.6	7.9
Principal
Gross	5,417.4	5,899.3
Ceded	(226.3)	(182.8)
Interest
Gross	2,804.2	2,601.6
Ceded	(74.9)	(70.9)
Total net outstanding exposure
Gross	8,221.6	8,500.9
Ceded	(301.2)	(253.7)
Expected cash flows
Gross	1,775.8	2,036.6
Ceded	(114.1)	(68.9)
Less: Potential recoveries
Gross	(811.4)	(533.0)
Ceded	29.4	16.6
Subtotal
Gross	964.4	1,503.6
Ceded	(84.7)	(52.3)
Discount
Gross	(345.5)	(613.2)
Ceded	35.4	21.5
Present value of expected cash flows
Gross	618.9	890.4
Ceded	(49.3)	(30.8)
Deferred premium revenue
Gross	394.7	572.4
Ceded	(23.7)	(30.3)
Reserves (salvage)
Gross	360.9	424.4
Ceded	(36.8)	(9.5)
BIG 3 | Financial Guaranty
Number of risks
Gross	127	115
Ceded	(50)	(42)
Remaining weighted average contract period
Gross (in years)	8.7	8.8
Ceded (in years)	6.0	6.0
Principal
Gross	7,844.2	7,954.5
Ceded	(651.7)	(673.6)
Interest
Gross	2,280.8	2,490.7
Ceded	(171.3)	(186.3)
Total net outstanding exposure
Gross	10,125.0	10,445.2
Ceded	(823.0)	(859.9)
Expected cash flows
Gross	2,208.9	2,256.6
Ceded	(137.8)	(133.2)
Less: Potential recoveries
Gross	(1,759.6)	(2,543.6)
Ceded	96.9	197.5
Subtotal
Gross	449.3	(287.0)
Ceded	(40.9)	64.3
Discount
Gross	(110.4)	(139.6)
Ceded	(3.8)	(7.9)
Present value of expected cash flows
Gross	338.9	(426.6)
Ceded	(44.7)	56.4
Deferred premium revenue
Gross	995.1	995.9
Ceded	(122.2)	(120.7)
Reserves (salvage)
Gross	(86.5)	(815.9)
Ceded	$ 9.1	$ 105.8