Fair Value Measurement (Details 5) (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
Carrying Amount
Assets:
Fixed maturity securities	10,204,900,000	10,141,900,000
Short-term investments	903,400,000	734,000,000
Other invested assets	154,100,000	170,400,000
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	169,000,000	180,200,000
Liabilities:
Financial guaranty insurance contracts	4,827,600,000	4,664,000,000
Long-term debt	1,034,700,000	1,038,300,000
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
Other liabilities	18,300,000	7,600,000
Estimated Fair Value
Assets:
Fixed maturity securities	10,204,900,000	10,141,900,000
Short-term investments	903,400,000	734,000,000
Other invested assets	157,700,000	182,400,000
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	169,000,000	180,200,000
Liabilities:
Financial guaranty insurance contracts	6,102,300,000	4,319,800,000
Long-term debt	1,265,600,000	1,186,300,000
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
Other liabilities	$ 18,300,000	$ 7,600,000