Fair Values of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 3,048.1		$ 3,048.1		$ 3,051.4
Embedded Derivative in Modified Coinsurance Arrangement	(3.3)	(4.8)	9.0	9.3
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	13.50%		13.50%
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	86.50%		86.50%
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	70.10%		70.10%
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	4.20%		4.20%
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	12.20%		12.20%
Policy Loans Ceded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 2,831.6		$ 2,831.6		$ 2,838.3