Fair Values of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 3,075.1		$ 3,075.1		$ 2,996.1
Embedded Derivative in Modified Coinsurance Arrangement	(33.7)	1.6	(24.4)	(3.7)
Policy Loans Ceded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 2,864.0		$ 2,864.0		$ 2,790.5
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Pricing Service
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of fixed maturity securities valued	15.70%		15.70%
Other Than Fair Value Inputs Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of fixed maturity securities valued	84.30%		84.30%
Significant Other Observable Inputs (Level 2) | Pricing Service
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of fixed maturity securities valued	68.70%		68.70%
Significant Other Observable Inputs (Level 2) | Other Observable Market Data
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of fixed maturity securities valued	4.40%		4.40%
Fair Value Inputs Level 2 Or Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of fixed maturity securities valued	11.20%		11.20%