Investments - Additional Information (Detail)		12 Months Ended
		Dec. 31, 2017 USD ($) Investment MortgageLoan LimitedPartnership Loan	Dec. 31, 2016 USD ($) Investment MortgageLoan LimitedPartnership Loan	Dec. 31, 2015 LimitedPartnership Loan
Schedule Of Available For Sale Securities [Line Items]
Fixed maturity AFS securities, at estimated fair value		$ 1,548,638,000	$ 1,487,037,000
Decrease in derivatives		41,153,000
Other financial assets, subject to offsetting		129,000	1,304,000
Other financial liabilities, subject to offsetting		$ 710,000	$ 916,000
Number of impaired limited partnerships | LimitedPartnership		0	0	0
Other Financial Liabilities
Schedule Of Available For Sale Securities [Line Items]
Other financial liabilities, subject to offsetting		$ 0	$ 0
Other Financial Assets
Schedule Of Available For Sale Securities [Line Items]
Other financial assets, subject to offsetting		$ 0	$ 0
Commercial mortgage loans
Schedule Of Available For Sale Securities [Line Items]
Concentrations of mortgage loans		82.00%
Commercial Mortgage Loans that had Two or more Payments Delinquent
Schedule Of Available For Sale Securities [Line Items]
Mortgage loans on real estate, number of loans | MortgageLoan		0	0
Estimate of Fair Value, Fair Value Disclosure
Schedule Of Available For Sale Securities [Line Items]
Assets		$ 634,232,000	$ 747,854,000
Corporate securities
Schedule Of Available For Sale Securities [Line Items]
Fixed maturity AFS securities, at estimated fair value		$ 942,024,000	$ 911,851,000
Number of non-income producing investment | Investment		1
Investment in non-income producing		$ 420,000
Mortgage loans on real estate | Estimate of Fair Value, Fair Value Disclosure
Schedule Of Available For Sale Securities [Line Items]
Total number of securities in an unrealized loss position | Investment		79	81
Number of investment securities | Investment		371	360
Prepayment premiums		$ 1,183,000	$ 0
Mortgage loans on real estate, number of loans | Loan		0	0	0
Assets	[1]	$ 26,049,000	$ 115,020,000
Deferred net gains (losses) on derivatives recorded in AOCI that are expected to be reclassified during the next 12 months		(1,100,000)
Deposited with insurance regulatory authorities
Schedule Of Available For Sale Securities [Line Items]
Investment securities, estimated fair value		4,246,000	13,371,000
Future
Schedule Of Available For Sale Securities [Line Items]
Collateral, fair value		2,488,000	5,276,000
BBB
Schedule Of Available For Sale Securities [Line Items]
Fixed maturity AFS securities, at estimated fair value		$ 120,663,000	$ 65,730,000

[1] The fair value of mortgage loans on real estate is estimated by discounting the future cash flows using the current rates at which similar loans would be made to borrowers with similar credit ratings and/or similar remaining maturities.