Fair Value Measurement (Details 2) (USD $)	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets:
Short-term investments	$ 734,046,000	$ 1,055,567,000
Other invested assets	222,869,000	283,032,000
Credit derivative assets	468,933,000	592,898,000
FG VIEs' assets, at fair value	2,819,077,000	3,657,481,000	1,925,300,000
Liabilities:
Credit derivative liabilities	1,772,803,000	2,462,831,000
FG VIEs' liabilities with recourse, at fair value	2,396,945,000	3,030,908,000	2,110,900,000
FG VIEs' liabilities without recourse, at fair value	1,061,497,000	1,337,214,000	226,000,000
Fair value measurement on recurring basis
Assets:
Total fixed maturity securities	10,141,900,000	9,402,300,000
Short-term investments	734,000,000	1,055,600,000
Other invested assets	43,500,000	33,300,000
Credit derivative assets	468,900,000	592,900,000
FG VIEs' assets, at fair value	2,819,100,000	3,657,500,000
Other assets	79,500,000	44,400,000
Total assets carried at fair value	14,286,900,000	14,786,000,000
Liabilities:
Credit derivative liabilities	1,772,800,000	2,462,800,000
FG VIEs' liabilities with recourse, at fair value	2,396,900,000	3,030,900,000
FG VIEs' liabilities without recourse, at fair value	1,061,500,000	1,337,200,000
Other liabilities		100,000
Total liabilities carried at fair value	5,231,200,000	6,831,000,000
Fair value measurement on recurring basis | U.S. government and agencies
Assets:
Total fixed maturity securities	922,400,000	1,048,200,000
Fair value measurement on recurring basis | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	5,455,400,000	4,959,900,000
Fair value measurement on recurring basis | Corporate securities
Assets:
Total fixed maturity securities	1,038,400,000	992,500,000
Fair value measurement on recurring basis | RMBS
Assets:
Total fixed maturity securities	1,427,900,000	1,117,100,000
Fair value measurement on recurring basis | CMBS
Assets:
Total fixed maturity securities	500,000,000	379,100,000
Fair value measurement on recurring basis | Asset-backed securities
Assets:
Total fixed maturity securities	458,100,000	502,900,000
Fair value measurement on recurring basis | Foreign government securities
Assets:
Total fixed maturity securities	339,700,000	348,600,000
Fair value measurement on recurring basis | Level 1
Assets:
Short-term investments	210,300,000	277,400,000
Other invested assets		200,000
Other assets	25,700,000	25,700,000
Total assets carried at fair value	236,000,000	303,300,000
Fair value measurement on recurring basis | Level 2
Assets:
Total fixed maturity securities	9,763,300,000	9,092,700,000
Short-term investments	523,700,000	778,200,000
Other invested assets	32,800,000	21,400,000
Other assets		18,700,000
Total assets carried at fair value	10,319,800,000	9,911,000,000
Liabilities:
Other liabilities		100,000
Total liabilities carried at fair value		100,000
Fair value measurement on recurring basis | Level 2 | U.S. government and agencies
Assets:
Total fixed maturity securities	922,400,000	1,048,200,000
Fair value measurement on recurring basis | Level 2 | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	5,445,900,000	4,959,900,000
Fair value measurement on recurring basis | Level 2 | Corporate securities
Assets:
Total fixed maturity securities	1,038,400,000	992,500,000
Fair value measurement on recurring basis | Level 2 | RMBS
Assets:
Total fixed maturity securities	1,294,300,000	1,071,700,000
Fair value measurement on recurring basis | Level 2 | CMBS
Assets:
Total fixed maturity securities	500,000,000	379,100,000
Fair value measurement on recurring basis | Level 2 | Asset-backed securities
Assets:
Total fixed maturity securities	222,600,000	292,700,000
Fair value measurement on recurring basis | Level 2 | Foreign government securities
Assets:
Total fixed maturity securities	339,700,000	348,600,000
Fair value measurement on recurring basis | Level 3
Assets:
Total fixed maturity securities	378,600,000	309,600,000
Other invested assets	10,700,000	11,700,000
Credit derivative assets	468,900,000	592,900,000
FG VIEs' assets, at fair value	2,819,100,000	3,657,500,000
Other assets	53,800,000
Total assets carried at fair value	3,731,100,000	4,571,700,000
Liabilities:
Credit derivative liabilities	1,772,800,000	2,462,800,000
FG VIEs' liabilities with recourse, at fair value	2,396,900,000	3,030,900,000
FG VIEs' liabilities without recourse, at fair value	1,061,500,000	1,337,200,000
Total liabilities carried at fair value	5,231,200,000	6,830,900,000
Fair value measurement on recurring basis | Level 3 | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	9,500,000
Fair value measurement on recurring basis | Level 3 | RMBS
Assets:
Total fixed maturity securities	133,600,000	99,400,000
Fair value measurement on recurring basis | Level 3 | Asset-backed securities
Assets:
Total fixed maturity securities	235,500,000	210,200,000
Fair value measurement on non-recurring basis | Level 3
Liabilities:
Mortgage loans recorded at fair value	$ 9,000,000	$ 9,400,000