Fair Values of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 2,948.8		$ 2,948.8		$ 2,996.1
Embedded Derivative in Modified Coinsurance Arrangement	(4.8)	(23.6)	9.3	(5.3)
Policy Loans Ceded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 2,741.7		$ 2,741.7		$ 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	16.80%		16.80%
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	83.20%		83.20%
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	67.30%		67.30%
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	4.80%		4.80%
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	11.10%		11.10%