Investments (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012 Fixed maturity securities	Mar. 31, 2011 Fixed maturity securities	Mar. 31, 2012 Short-term investments.	Mar. 31, 2011 Short-term investments.	Mar. 31, 2012 Assets Acquired in Refinancing Transactions	Mar. 31, 2011 Assets Acquired in Refinancing Transactions	Mar. 31, 2012 Fixed, short-term maturity securities and assets acquired in refinancing transactions	Dec. 31, 2011 Fixed, short-term maturity securities and assets acquired in refinancing transactions
Investments
Accrued investment income									$ 101,000,000	$ 100,700,000
Net Investment Income
Gross investment income	100,100,000	100,100,000	98,900,000	98,500,000	200,000	300,000	1,000,000	1,300,000
Investment expenses	(2,300,000)	(2,700,000)
Net investment income	97,762,000	97,412,000
Net Realized Investment Gains (Losses)
Realized gains on investment portfolio	9,500,000	9,900,000
Realized losses on investment portfolio	(3,300,000)	(2,500,000)
Other-than-temporary impairment (OTTI):
Intent to sell	(300,000)	(2,700,000)
Credit component of OTTI securities	(4,600,000)	(1,900,000)
OTTI	(4,900,000)	(4,600,000)
Net realized investment gains (losses)	1,316,000	2,806,000
Roll Forward of Credit Losses in the Investment Portfolio
Balance, at the beginning of the period	46,700,000	27,300,000
Additions for credit losses on securities for which an OTTI was not previously recognized	2,100,000	1,400,000
Additions for credit losses on securities for which an OTTI was previously recognized	2,500,000	500,000
Balance, at the end of the period	$ 51,300,000	$ 29,200,000