Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Gain (Loss) on Embedded Derivative	$ (0.8)	$ (2.3)	$ 4.7	$ (4.0)
Ceded Policy Loans	$ 3,348.0		$ 3,348.0		$ 3,449.3
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	76.40%		76.40%
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	9.60%		9.60%
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	23.60%		23.60%
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	63.10%		63.10%
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	3.70%		3.70%