Investments (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Net Investment Income
Gross investment income	$ 103,100,000	$ 92,600,000	$ 202,200,000	$ 179,400,000
Investment expenses	(2,300,000)	(1,700,000)	(5,000,000)	(4,200,000)
Net investment income	100,827,000	90,871,000	197,214,000	175,173,000	354,700,000	259,200,000	162,600,000
Accrued investment income	103,800,000		103,800,000		97,900,000
Net Realized Investment Gains (Losses)
Realized gains on investment portfolio	10,800,000	12,100,000	20,800,000	25,300,000
Realized losses on investment portfolio	(4,300,000)	(3,200,000)	(6,900,000)	(6,500,000)
Other-than-temporary impairment (OTTI):
Intent to sell	(800,000)	(1,300,000)	(3,500,000)	(1,700,000)
Credit component of OTTI securities	(10,800,000)	(16,000,000)	(12,700,000)	(16,100,000)
OTTI	(11,600,000)	(17,300,000)	(16,200,000)	(17,800,000)
Net realized investment gains (losses)	(5,090,000)	(8,438,000)	(2,284,000)	975,000	(2,000,000)	(32,700,000)	(69,800,000)
Full unrealized loss	26,818,000	17,412,000	33,765,000	18,529,000
Roll Forward of Credit Losses in the Investment Portfolio
Balance, at the beginning of the period	29,200,000	20,000,000	27,300,000	19,900,000	19,900,000
Additions for credit losses on securities for which an OTTI was not previously recognized	9,100,000		10,500,000
Eliminations of securities issued by VIE	(13,500,000)		(13,500,000)
Reductions for securities sold during the period	(5,000,000)		(5,000,000)
Additions for credit losses on securities for which an OTTI was previously recognized	1,800,000		2,300,000	100,000
Balance, at the end of the period	21,600,000	20,000,000	21,600,000	20,000,000	27,300,000	19,900,000
Fixed maturity securities
Net Investment Income
Gross investment income	102,800,000	92,600,000	201,600,000	179,800,000
Short-term investments.
Net Investment Income
Gross investment income	$ 300,000		$ 600,000	$ (400,000)