Fair Value of Financial Instruments Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 3,242.5		$ 3,242.5		$ 3,133.8
Long-term Debt	2,631.3		2,631.3		2,755.4
Gain (Loss) on Embedded Derivative	(0.3)	19.7	18.5	28.7
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	89.70%		89.70%
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt	2,163.9		2,163.9		1,212.0
Fair Value, Inputs, Level 1 | Pricing Service
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of Total Fair Value of Fixed Maturities Securities	10.30%		10.30%
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt	663.0		663.0		1,756.8
Fair Value, Inputs, Level 2 | Pricing Service
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of Total Fair Value of Fixed Maturities Securities	73.00%		73.00%
Fair Value, Inputs, Level 2 | 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	3.70%		3.70%
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt	0.6		0.6		0
Fair Value Inputs Level 2 Or Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of Total Fair Value of Fixed Maturities Securities	13.00%		13.00%
Ceded Policy Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 3,014.3		$ 3,014.3		$ 2,912.7