Fair Values of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 3,117.0		$ 3,117.0		$ 3,051.4
Embedded Derivative in Modified Coinsurance Arrangement	19.7	(33.7)	28.7	(24.4)
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	472.7		472.7
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	15.30%		15.30%
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	84.70%		84.70%
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	2,455.9		2,455.9
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	68.70%		68.70%
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	3.30%		3.30%
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	12.70%		12.70%
Policy Loans Ceded
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 2,897.5		$ 2,897.5		$ 2,838.3