Fair Value Measurement (Details 2) (USD $)	Mar. 31, 2012	Dec. 31, 2011
Assets:
Short-term investments	$ 903,363,000	$ 734,046,000
Other invested assets	203,900,000	222,869,000
Credit derivative assets	463,556,000	468,933,000
FG VIEs' assets, at fair value	2,827,652,000	2,819,077,000
Other assets	67,900,000	79,500,000
Liabilities:
Credit derivative liabilities	2,416,268,000	1,772,803,000
FG VIEs' liabilities with recourse, at fair value	2,365,177,000	2,396,945,000
FG VIEs' liabilities without recourse, at fair value	1,085,618,000	1,061,497,000
Fair value measurement on recurring basis
Assets:
Total fixed maturity securities	10,204,900,000	10,141,900,000
Short-term investments	903,400,000	734,000,000
Other invested assets	68,500,000	43,500,000
Credit derivative assets	463,600,000	468,900,000
FG VIEs' assets, at fair value	2,827,700,000	2,819,100,000
Other assets	67,900,000	79,500,000
Total assets carried at fair value	14,536,000,000	14,286,900,000
Liabilities:
Credit derivative liabilities	2,416,300,000	1,772,800,000
FG VIEs' liabilities with recourse, at fair value	2,365,200,000	2,396,900,000
FG VIEs' liabilities without recourse, at fair value	1,085,600,000	1,061,500,000
Total liabilities carried at fair value	5,867,100,000	5,231,200,000
Fair value measurement on recurring basis | U.S. government and agencies
Assets:
Total fixed maturity securities	866,700,000	922,400,000
Fair value measurement on recurring basis | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	5,526,100,000	5,455,400,000
Fair value measurement on recurring basis | Corporate securities
Assets:
Total fixed maturity securities	1,042,800,000	1,038,400,000
Fair value measurement on recurring basis | RMBS
Assets:
Total fixed maturity securities	1,424,000,000	1,427,900,000
Fair value measurement on recurring basis | CMBS
Assets:
Total fixed maturity securities	501,500,000	500,000,000
Fair value measurement on recurring basis | Asset-backed securities
Assets:
Total fixed maturity securities	482,900,000	458,100,000
Fair value measurement on recurring basis | Foreign government securities
Assets:
Total fixed maturity securities	360,900,000	339,700,000
Fair value measurement on recurring basis | Level 2
Assets:
Total fixed maturity securities	9,801,600,000	9,763,300,000
Short-term investments	683,100,000	523,700,000
Other invested assets	59,000,000	32,800,000
Total assets carried at fair value	10,543,700,000	10,319,800,000
Fair value measurement on recurring basis | Level 2 | U.S. government and agencies
Assets:
Total fixed maturity securities	866,700,000	922,400,000
Fair value measurement on recurring basis | Level 2 | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	5,516,600,000	5,445,900,000
Fair value measurement on recurring basis | Level 2 | Corporate securities
Assets:
Total fixed maturity securities	1,042,800,000	1,038,400,000
Fair value measurement on recurring basis | Level 2 | RMBS
Assets:
Total fixed maturity securities	1,288,300,000	1,294,300,000
Fair value measurement on recurring basis | Level 2 | CMBS
Assets:
Total fixed maturity securities	501,500,000	500,000,000
Fair value measurement on recurring basis | Level 2 | Asset-backed securities
Assets:
Total fixed maturity securities	224,800,000	222,600,000
Fair value measurement on recurring basis | Level 2 | Foreign government securities
Assets:
Total fixed maturity securities	360,900,000	339,700,000
Fair value measurement on recurring basis | Level 3
Financial instruments carried at fair value
Notes		1,600,000
Assets:
Total fixed maturity securities	403,300,000	378,600,000
Other invested assets	9,400,000	10,700,000
Credit derivative assets	463,600,000	468,900,000
FG VIEs' assets, at fair value	2,827,700,000	2,819,100,000
Other assets	39,900,000	53,800,000
Total assets carried at fair value	3,743,900,000	3,731,100,000
Liabilities:
Credit derivative liabilities	2,416,300,000	1,772,800,000
FG VIEs' liabilities with recourse, at fair value	2,365,200,000	2,396,900,000
FG VIEs' liabilities without recourse, at fair value	1,085,600,000	1,061,500,000
Total liabilities carried at fair value	5,867,100,000	5,231,200,000
Fair value measurement on recurring basis | Level 3 | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	9,500,000	9,500,000
Fair value measurement on recurring basis | Level 3 | RMBS
Assets:
Total fixed maturity securities	135,700,000	133,600,000
Fair value measurement on recurring basis | Level 3 | Asset-backed securities
Assets:
Total fixed maturity securities	258,100,000	235,500,000
Fair value measurement on recurring basis | Level 1
Assets:
Short-term investments	220,300,000	210,300,000
Other invested assets	100,000
Other assets	28,000,000	25,700,000
Total assets carried at fair value	248,400,000	236,000,000
Fair value measurement on non-recurring basis | Level 3
Liabilities:
Mortgage loans recorded at fair value	$ 7,800,000	$ 9,000,000