Investments (Details) (USD $)	3 Months Ended								12 Months Ended				12 Months Ended			1 Months Ended				12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Maximum	Dec. 31, 2011 Minimum	Dec. 31, 2009 Retained Earnings	Dec. 31, 2009 Accumulated Other Comprehensive Income	Apr. 30, 2009 Cumulative effect of the adoption of the OTTI standard Retained Earnings	Apr. 30, 2009 Cumulative effect of the adoption of the OTTI standard Accumulated Other Comprehensive Income	Dec. 31, 2011 Investment in Portfolio Funding Company LLC I	Sep. 30, 2010 Investment in Portfolio Funding Company LLC I	Dec. 31, 2011 Fixed maturity securities	Dec. 31, 2010 Fixed maturity securities	Dec. 31, 2009 Fixed maturity securities	Dec. 31, 2011 Short-term investments.	Dec. 31, 2010 Short-term investments.	Dec. 31, 2009 Short-term investments.	Dec. 31, 2011 Fixed and short-term maturity securities	Dec. 31, 2010 Fixed and short-term maturity securities
Investments
Percentage of fixed maturity securities classified as available-for-sale																										98.00%
Maturity period for classification as short-term investments (in years)												1 year
Percentage of an equity investment acquired in a restructuring of an insured CDS																		50.00%	50.00%
Reclassification of losses from retained earnings to accumulated OCI (AOCI)														$ (57,652,000)	$ 57,652,000	$ (62,200,000)	$ 62,200,000
Percentage of decline in market value of a security below amortized cost considered for assessment of impairment of investments													20.00%
Continuous period of more than 20% decline in market value considered for assessment of impairment of investments (in months)													6 months
Continuous period of any percentage decline in market value considered for assessment of impairment of investments (in months)									12 months
Accrued investment income																										100,300,000	97,900,000
Net Investment Income
Gross investment income									400,100,000	363,200,000	265,600,000									399,200,000	359,700,000	262,400,000	900,000	3,500,000	3,200,000
Investment expenses									(9,100,000)	(8,500,000)	(6,400,000)
Net investment income	100,300,000	93,500,000	101,100,000	96,100,000	93,900,000	85,600,000	90,900,000	84,300,000	391,017,000	354,703,000	259,222,000
Net Realized Investment Gains (Losses)
Realized gains on investment portfolio									36,400,000	31,100,000	28,300,000
Realized losses on investment portfolio									(9,800,000)	(5,700,000)	(15,200,000)
Other-than-temporary impairment (OTTI):
Intent to sell									(5,400,000)	(4,000,000)	(13,400,000)
Credit component of OTTI securities									(39,200,000)	(23,400,000)	(32,400,000)
OTTI									(44,600,000)	(27,400,000)	(45,800,000)
Net realized investment gains (losses)	(4,600,000)	(11,100,000)	(5,100,000)	2,800,000	(600,000)	(2,400,000)	(8,400,000)	9,400,000	(17,992,000)	(1,994,000)	(32,662,000)
Roll Forward of Credit Losses in the Investment Portfolio
Balance, at the beginning of the period				27,300,000				19,900,000	27,300,000	19,900,000	600,000
Additions for credit losses on securities for which an OTTI was not previously recognized									26,600,000	7,300,000	13,600,000
Eliminations of securities issued by VIE									(13,500,000)
Reductions for securities sold during the period									(6,400,000)		(100,000)
Additions for credit losses on securities for which an OTTI was previously recognized									12,700,000	100,000	6,100,000
Reductions for credit losses now recognized in earnings due to intention to sell the security											(300,000)
Balance, at the end of the period	$ 46,700,000				$ 27,300,000				$ 46,700,000	$ 27,300,000	$ 19,900,000