Investments (Details 5) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Amortized Cost
Due within one year	$ 279,400,000
Due after one year through five years	1,636,100,000
Due after five years through 10 years	2,515,400,000
Due after 10 years	3,457,600,000
Total	9,596,052,000	9,289,444,000
Estimated Fair Value
Due within one year	281,200,000
Due after one year through five years	1,685,500,000
Due after five years through 10 years	2,626,000,000
Due after 10 years	3,537,900,000
Total	9,864,203,000	9,415,315,000
Fixed maturity securities held under trust for the benefit of reinsured companies	374,100,000	365,300,000
Eligible securities deposited for the benefit of policyholders	19,100,000	19,200,000
Fair market value of company's pledged securities	768,600,000	765,900,000
Fair value of the securities purchased to mitigate insured losses	168,100,000
Par of the securities purchased to mitigate insured losses	406,600,000
Fair value of the securities purchased under derivative contracts	193,400,000
Par of the securities purchased under derivative contracts	246,900,000
RMBS
Amortized Cost
Total	1,208,800,000
Estimated Fair Value
Total	1,217,500,000
CMBS
Amortized Cost
Total	498,800,000
Estimated Fair Value
Total	$ 516,100,000