Investments (Details 5) (USD $)	Sep. 30, 2011	Dec. 31, 2010
Amortized Cost
Due within one year	$ 285,200,000
Due after one year through five years	1,700,700,000
Due after five years through 10 years	2,425,900,000
Due after 10 years	3,405,800,000
Total	9,650,310,000	9,274,718,000
Estimated Fair Value
Due within one year	283,900,000
Due after one year through five years	1,760,500,000
Due after five years through 10 years	2,605,800,000
Due after 10 years	3,603,000,000
Total	10,091,241,000	9,402,287,000
Fixed maturity securities held under trust for the benefit of reinsured companies	406,100,000	365,300,000
Eligible securities deposited for the benefit of policyholders	23,100,000	19,200,000
Fair market value of company's pledged securities	774,300,000	765,900,000
Fair value of the securities purchased to mitigate insured losses	148,000,000
Par of the securities purchased to mitigate insured losses	473,800,000
Fair value of the securities where the Company has obtained the referenced obligations on derivative contracts	194,000,000
Par of the securities where the Company has obtained the referenced obligations on derivative contracts	245,400,000
RMBS
Amortized Cost
Total	1,341,900,000
Estimated Fair Value
Total	1,329,600,000
CMBS
Amortized Cost
Total	490,800,000
Estimated Fair Value
Total	$ 508,400,000