Fair Value Measurement (Details 4) (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
Carrying Amount
Assets:
Fixed maturity securities	10,141,900,000	9,402,300,000
Short-term investments	734,000,000	1,055,600,000
Other invested assets	170,400,000	259,800,000
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
Liabilities:
Financial guaranty insurance contracts	4,664,000,000	4,777,600,000
Long-term debt	1,038,300,000	1,052,900,000
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
Estimated Fair Value
Assets:
Fixed maturity securities	10,141,900,000	9,402,300,000
Short-term investments	734,000,000	1,055,600,000
Other invested assets	182,400,000	269,700,000
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	79,500,000	44,400,000
Liabilities:
Financial guaranty insurance contracts	4,319,800,000	5,582,800,000
Long-term debt	1,186,300,000	1,201,800,000
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