Investments (Details 5) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Amortized Cost
Due within one year	$ 391,300,000
Due after one year through five years	1,543,000,000
Due after five years through 10 years	2,372,200,000
Due after 10 years	3,402,100,000
Total	9,638,404,000	9,274,718,000
Estimated Fair Value
Due within one year	391,100,000
Due after one year through five years	1,599,600,000
Due after five years through 10 years	2,592,000,000
Due after 10 years	3,631,300,000
Total	10,141,850,000	9,402,287,000
Fixed maturity securities held under trust for the benefit of reinsured companies	380,100,000	365,300,000
Fixed maturity investments in a segregated portfolio primarily to provide collateral for a LOC	3,500,000
Eligible securities deposited for the benefit of policyholders	23,900,000	19,200,000
Fair market value of company's pledged securities	779,900,000	765,900,000
Fair value of the securities purchased to mitigate insured losses	192,300,000
Par of the securities purchased to mitigate insured losses	701,300,000
Fair value of the securities purchased under derivative contracts	186,000,000
Par of the securities purchased under derivative contracts	222,400,000
RMBS
Amortized Cost
Total	1,454,300,000
Estimated Fair Value
Total	1,427,900,000
CMBS
Amortized Cost
Total	475,600,000
Estimated Fair Value
Total	$ 500,000,000