Fair Value of Financial Instruments Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Ceded Policy Loans	$ 3,307.5	$ 3,206.1
Long-term Debt	2,738.4	2,999.4
Gain (Loss) on Embedded Derivative	$ 30.8	40.9	$ (37.7)
Other Than Fair Value Inputs Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Percentage of Total Fair Value of Fixed Maturities Securities	90.80%
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term Debt	$ 1,171.8	709.8
Percentage of Total Fair Value of Fixed Maturities Securities	9.20%
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-term Debt	$ 1,876.9	$ 2,466.0
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	77.60%
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	2.50%
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	10.70%
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