Fair Values of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 3,051.4	$ 2,996.1
Embedded Derivative in Modified Coinsurance Arrangement	(39.4)	21.1	243.1
Fair Value, Inputs, Level 1 [Member] | Pricing Service
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of fixed maturity securities valued	10.90%
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	89.10%
Fair Value, Inputs, Level 2 [Member] | Pricing Service
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of fixed maturity securities valued	71.10%
Fair Value, Inputs, Level 2 [Member] | Other Observable Market Data
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of fixed maturity securities valued	4.10%
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	13.90%
Policy Loans Ceded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 2,838.3	$ 2,790.5