Fair Value Measurement (Details 5) (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
Carrying Amount
Assets:
Fixed maturity securities	10,207,500,000		10,141,900,000
Short-term investments	919,800,000		734,000,000
Other invested assets	153,600,000		170,400,000
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	243,300,000		185,600,000
Liabilities:
Financial guaranty insurance contracts	4,663,200,000	[1]	4,664,000,000	[1]
Long-term debt	846,400,000	[2]	1,038,300,000	[2]
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
Other liabilities	93,600,000		15,700,000
Estimated Fair Value
Assets:
Fixed maturity securities	10,207,500,000		10,141,900,000
Short-term investments	919,800,000		734,000,000
Other invested assets	165,800,000		182,400,000
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	243,300,000		185,600,000
Liabilities:
Financial guaranty insurance contracts	5,555,200,000	[1]	4,319,800,000	[1]
Long-term debt	1,027,000,000	[2]	1,186,300,000	[2]
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
Other liabilities	$ 93,600,000		$ 15,700,000

[1] Carrying amount includes the balance sheet amounts related to financial guaranty insurance contract premiums and losses, net of reinsurance.

[2] Carrying amount represented principal less accumulated discount or plus accumulated premium.