Investments (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Net Investment Income
Gross investment income	$ 96,100,000	$ 88,000,000	$ 298,300,000	$ 267,400,000
Investment expenses	(2,600,000)	(2,400,000)	(7,600,000)	(6,600,000)
Net investment income	93,534,000	85,615,000	290,748,000	260,788,000
Accrued investment income	100,400,000		100,400,000		97,900,000
Net Realized Investment Gains (Losses)
Realized gains on investment portfolio	5,700,000	3,200,000	26,500,000	28,500,000
Realized losses on investment portfolio	(400,000)	(1,000,000)	(7,300,000)	(7,500,000)
Other-than-temporary impairment (OTTI):
Intent to sell	(1,400,000)	(300,000)	(4,900,000)	(2,000,000)
Credit component of OTTI securities	(15,000,000)	(4,300,000)	(27,700,000)	(20,400,000)
OTTI	(16,400,000)	(4,600,000)	(32,600,000)	(22,400,000)
Net realized investment gains (losses)	(11,085,000)	(2,374,000)	(13,369,000)	(1,399,000)
Roll Forward of Credit Losses in the Investment Portfolio
Balance, at the beginning of the period	21,600,000	20,000,000	27,300,000	19,900,000
Additions for credit losses on securities for which an OTTI was not previously recognized	14,400,000	4,300,000	24,900,000	4,300,000
Eliminations of securities issued by VIE			(13,500,000)
Reductions for securities sold during the period			(5,000,000)
Additions for credit losses on securities for which an OTTI was previously recognized	500,000		2,800,000	100,000
Balance, at the end of the period	36,500,000	24,300,000	36,500,000	24,300,000
Fixed maturity securities
Net Investment Income
Gross investment income	95,900,000	87,500,000	297,500,000	267,300,000
Short-term investments.
Net Investment Income
Gross investment income	$ 200,000	$ 500,000	$ 800,000	$ 100,000