Summary of Variable Annuity Contracts with Guarantees (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Guaranteed death benefits
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account value		$ 274.3	$ 273.6
Net amount at risk	[1]	$ 6.9	$ 8.5
Average attained age of contract holders		67 years	66 years
Liability for guarantees related to income benefits
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account value		$ 20.5	$ 19.8
Net amount at risk	[2]	$ 2.1	$ 2.5
Weighted average waiting period until annuitization or guarantee date		0 years	0 years
Withdrawal benefit
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account value		$ 17.4	$ 18.6
Net amount at risk	[3]	0.4	0.4
Guaranteed accumulation benefits
Net Amount at Risk by Product and Guarantee [Line Items]
Separate account value		32.3	65.3
Net amount at risk	[4]	$ 0.4	$ 1.4
Weighted average waiting period until annuitization or guarantee date		4 years	2 years

[1] Defined as the estimated current guaranteed minimum death benefit in excess of the current account balance as of the balance sheet date.

[2] Defined as the estimated present value of the guaranteed minimum annuity payments in excess of the current account balance.

[3] Defined as the estimated current guaranteed minimum withdrawal balance (initial deposit) in excess of the current account balance as of the balance sheet date.

[4] Defined as the estimated present value of the guaranteed minimum accumulation balance in excess of the current account balance.