Fair Value Measurement (Details 2) (USD $)	Jun. 30, 2012		Dec. 31, 2011
Assets:
Short-term investments	$ 919,784,000		$ 734,046,000
Other invested assets	194,447,000		222,869,000
Credit derivative assets	429,891,000		468,933,000
FG VIEs' assets, at fair value	2,725,979,000		2,819,077,000
Liabilities:
Credit derivative liabilities	2,095,852,000		1,772,803,000
FG VIEs' liabilities with recourse, at fair value	2,239,067,000		2,396,945,000
FG VIEs' liabilities without recourse, at fair value	1,042,275,000		1,061,497,000
Fair value measurement on recurring basis | Estimated Fair Value
Assets:
Total fixed maturity securities	10,207,500,000		10,141,900,000
Short-term investments	919,800,000		734,000,000
Other invested assets	85,200,000	[1]	43,500,000	[1]
Credit derivative assets	429,900,000		468,900,000
FG VIEs' assets, at fair value	2,726,000,000		2,819,100,000
Other assets	67,200,000	[2]	79,500,000	[2]
Total assets carried at fair value	14,435,600,000		14,286,900,000
Liabilities:
Credit derivative liabilities	2,095,900,000
FG VIEs' liabilities with recourse, at fair value	2,239,000,000
FG VIEs' liabilities without recourse, at fair value	1,042,300,000
Total liabilities carried at fair value	5,377,200,000		5,231,200,000
Fair value measurement on recurring basis | Estimated Fair Value | U.S. government and agencies
Assets:
Total fixed maturity securities	828,100,000		922,400,000
Fair value measurement on recurring basis | Estimated Fair Value | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	5,676,700,000		5,455,400,000
Fair value measurement on recurring basis | Estimated Fair Value | Corporate securities
Assets:
Total fixed maturity securities	1,032,100,000		1,038,400,000
Fair value measurement on recurring basis | Estimated Fair Value | RMBS
Assets:
Total fixed maturity securities	1,395,600,000		1,427,900,000
Fair value measurement on recurring basis | Estimated Fair Value | CMBS
Assets:
Total fixed maturity securities	486,500,000		500,000,000
Fair value measurement on recurring basis | Estimated Fair Value | Asset-backed securities
Assets:
Total fixed maturity securities	488,300,000		458,100,000
Fair value measurement on recurring basis | Estimated Fair Value | Foreign government securities
Assets:
Total fixed maturity securities	300,200,000		339,700,000
Fair value measurement on recurring basis | Level 2
Assets:
Total fixed maturity securities	9,757,100,000		9,763,300,000
Short-term investments	857,000,000		523,700,000
Other invested assets	75,400,000	[1]	32,800,000	[1]
Credit derivative assets	0
FG VIEs' assets, at fair value	0
Other assets	0	[2]
Total assets carried at fair value	10,689,500,000		10,319,800,000
Liabilities:
Credit derivative liabilities	0		0
FG VIEs' liabilities with recourse, at fair value	0		0
FG VIEs' liabilities without recourse, at fair value	0		0
Total liabilities carried at fair value	0		0
Other Investments Fair Value Disclosure	76,800,000
Fair value measurement on recurring basis | Level 2 | U.S. government and agencies
Assets:
Total fixed maturity securities	828,100,000		922,400,000
Fair value measurement on recurring basis | Level 2 | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	5,667,000,000		5,445,900,000
Fair value measurement on recurring basis | Level 2 | Corporate securities
Assets:
Total fixed maturity securities	1,032,100,000		1,038,400,000
Fair value measurement on recurring basis | Level 2 | RMBS
Assets:
Total fixed maturity securities	1,228,900,000		1,294,300,000
Fair value measurement on recurring basis | Level 2 | CMBS
Assets:
Total fixed maturity securities	486,500,000		500,000,000
Fair value measurement on recurring basis | Level 2 | Asset-backed securities
Assets:
Total fixed maturity securities	214,300,000		222,600,000
Fair value measurement on recurring basis | Level 2 | Foreign government securities
Assets:
Total fixed maturity securities	300,200,000		339,700,000
Fair value measurement on recurring basis | Level 3
Assets:
Total fixed maturity securities	450,400,000		378,600,000
Short-term investments	0
Other invested assets	9,800,000	[1]	10,700,000	[1]
Credit derivative assets	429,900,000		468,900,000
FG VIEs' assets, at fair value	2,726,000,000		2,819,100,000
Other assets	44,200,000	[2]	53,800,000	[2]
Total assets carried at fair value	3,660,300,000		3,731,100,000
Liabilities:
Credit derivative liabilities	2,095,900,000		1,772,800,000
FG VIEs' liabilities with recourse, at fair value	2,239,000,000		2,396,900,000
FG VIEs' liabilities without recourse, at fair value	1,042,300,000		1,061,500,000
Total liabilities carried at fair value	5,377,200,000		5,231,200,000
Mortgage loans recorded at fair value	8,400,000
Other Investments Fair Value Disclosure	1,400,000
Fair value measurement on recurring basis | Level 3 | U.S. government and agencies
Assets:
Total fixed maturity securities	0
Fair value measurement on recurring basis | Level 3 | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	9,700,000		9,500,000
Fair value measurement on recurring basis | Level 3 | Corporate securities
Assets:
Total fixed maturity securities	0
Fair value measurement on recurring basis | Level 3 | RMBS
Assets:
Total fixed maturity securities	166,700,000		133,600,000
Fair value measurement on recurring basis | Level 3 | CMBS
Assets:
Total fixed maturity securities	0
Fair value measurement on recurring basis | Level 3 | Asset-backed securities
Assets:
Total fixed maturity securities	274,000,000		235,500,000
Fair value measurement on recurring basis | Level 3 | Foreign government securities
Assets:
Total fixed maturity securities	0		0
Fair value measurement on recurring basis | Level 1
Assets:
Total fixed maturity securities	0
Short-term investments	62,800,000		210,300,000
Other invested assets	0	[1]	0	[1]
Credit derivative assets	0
FG VIEs' assets, at fair value	0
Other assets	23,000,000	[2]	25,700,000	[2]
Total assets carried at fair value	85,800,000		236,000,000
Liabilities:
Credit derivative liabilities	0		0
FG VIEs' liabilities with recourse, at fair value	0		0
FG VIEs' liabilities without recourse, at fair value	0		0
Total liabilities carried at fair value	0		0
Fair value measurement on recurring basis | Level 1 | U.S. government and agencies
Assets:
Total fixed maturity securities	0
Fair value measurement on recurring basis | Level 1 | Obligations of state and political subdivisions
Assets:
Total fixed maturity securities	0
Fair value measurement on recurring basis | Level 1 | Corporate securities
Assets:
Total fixed maturity securities	0
Fair value measurement on recurring basis | Level 1 | RMBS
Assets:
Total fixed maturity securities	0
Fair value measurement on recurring basis | Level 1 | CMBS
Assets:
Total fixed maturity securities	0
Fair value measurement on recurring basis | Level 1 | Asset-backed securities
Assets:
Total fixed maturity securities	0
Fair value measurement on recurring basis | Level 1 | Foreign government securities
Assets:
Total fixed maturity securities	0
Fair value measurement on non-recurring basis | Level 3
Liabilities:
Mortgage loans recorded at fair value	$ 8,400,000		$ 9,000,000

[1] Includes mortgage loans that are recorded at fair value on a non-recurring basis. At June 30, 2012 and December 31, 2011, such investments were carried at their market value of $8.4 million and $9.0 million, respectively.

[2] Includes fair value of CCS and supplemental executive retirement plan assets.