Investments - Additional Information (Detail) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2017 USD ($) Investment MortgageLoan	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) Investment MortgageLoan
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, at estimated fair value		$ 1,463,444		$ 1,487,037
Future
Schedule of Available-for-sale Securities [Line Items]
Collateral, fair value		$ 5,356		$ 5,276
Commercial mortgage loans
Schedule of Available-for-sale Securities [Line Items]
Concentrations of mortgage loans		90.00%
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		$ 752,124		$ 747,854
Estimate of Fair Value, Fair Value Disclosure | Mortgage loans on real estate
Schedule of Available-for-sale Securities [Line Items]
Total number of securities in an unrealized loss position | Investment		63		81
Number of investment securities | Investment		357		360
Prepayment premiums		$ 62	$ 0
Assets	[1]	124,725		$ 115,020
Deferred net (losses) on derivatives recorded in AOCI that are expected to be reclassified during the next 12 months		(988)
Standard & Poor's, BBB rating
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity available-for-sale securities, at estimated fair value		$ 68,561		$ 65,730

[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.