Fair Value of Financial Instruments Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Carrying amount of policy loans	$ 3,355.5		$ 3,355.5		$ 3,276.0
Long-term Debt	2,783.3		2,783.3		2,612.0
Gain (Loss) on Embedded Derivative	(2.3)	(0.3)	18.6	18.5
Fair Value Inputs Other Than Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Percentage of Total Fair Value of Fixed Maturities Securities	83.10%		83.10%
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term Debt	1,584.8		1,584.8		1,329.2
Percentage of Total Fair Value of Fixed Maturities Securities	16.90%		16.90%
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term Debt	1,472.9		1,472.9		1,495.2
Fair Value, Inputs, Level 2 | Pricing Service
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Percentage of Total Fair Value of Fixed Maturities Securities	68.40%		68.40%
Fair Value, Inputs, Level 2 | Other Observable Market Data
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Percentage of Total Fair Value of Fixed Maturities Securities	3.20%		3.20%
Fair Value Inputs Level 2 Or Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Percentage of Total Fair Value of Fixed Maturities Securities	11.50%		11.50%
Ceded Policy Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Carrying amount of policy loans	$ 3,119.3		$ 3,119.3		$ 3,043.7
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Private Equity Partnership Future Liquidation Term			1 year
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Private Equity Partnership Future Liquidation Term			10 years