Fair Value of Financial Instruments Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Ceded Policy Loans	$ 3,165.6		$ 3,165.6		$ 3,068.4
Long-term Debt	2,217.1		2,217.1		2,628.7
Gain (Loss) on Embedded Derivative	$ (27.3)	$ (2.3)	$ (33.2)	$ 18.6
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	84.10%		84.10%
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term Debt	$ 373.5		$ 373.5
Long-term Debt	$ 426.2		$ 426.2		849.7
Percentage of Total Fair Value of Fixed Maturities Securities	15.90%		15.90%
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Short-term Debt	$ 152.9		$ 152.9		158.9
Long-term Debt	$ 2,004.7		$ 2,004.7		$ 2,062.9
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	69.00%		69.00%
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	3.30%		3.30%
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	11.80%		11.80%
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			12 years