Fair Value of Financial Instruments Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 3,141.4		$ 3,133.8
Long-term Debt	2,673.9		2,755.4
Embedded Derivative, Gain (Loss) on Embedded Derivative, Net	6.4	12.3
Other Than Fair Value Inputs Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of Total Fair Value of Fixed Maturities Securities	90.50%
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt	1,241.2		1,212.0
Fair Value, Inputs, Level 1 [Member] | Pricing Service [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of Total Fair Value of Fixed Maturities Securities	9.50%
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt	1,713.7		1,756.8
Fair Value, Inputs, Level 2 [Member] | Pricing Service [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of Total Fair Value of Fixed Maturities Securities	73.40%
Fair Value, Inputs, Level 2 [Member] | Other Observable Market Data
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of Total Fair Value of Fixed Maturities Securities	3.40%
Fair Value Inputs Level 2 Or Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of Total Fair Value of Fixed Maturities Securities	13.70%
Ceded Policy Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 2,919.8		$ 2,912.7