Investments - A Summary of the Company's Realized Gains and Losses on Sales, Calls or Redemptions of Investments (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Proceeds from sales									$ 15,028	$ 27,764	$ 59,339
Gross realized investment gains									239	486	891
Gross realized investment (losses)									(1,354)	(5)	(1,728)
Net realized investment gains (losses)									(1,115)	481	(837)
Other, including gains (losses) on calls and redemptions									(421)	(1,128)	343
Net realized gains (losses) on investments	$ (65)	$ (329)	$ (1,111)	$ (31)	$ 114	$ (192)	$ (388)	$ (181)	(1,536)	(647)	(494)
Fixed Maturity Securities Available for Sale [Member]
Investment [Line Items]
Proceeds from sales									15,025	14,591	54,730
Gross realized investment gains									238	485	823
Gross realized investment (losses)									(1,354)	(5)	(1,728)
Net realized investment gains (losses)									(1,116)	480	(905)
Other, including gains (losses) on calls and redemptions									(144)	(520)	274
Net realized gains (losses) on investments									(1,260)	(40)	(631)
Equity Securities [Member]
Investment [Line Items]
Proceeds from sales									3	1
Gross realized investment gains									1	1
Net realized investment gains (losses)									1	1
Net realized gains (losses) on investments									1	1
Other [Member]
Investment [Line Items]
Proceeds from sales										13,172	4,609
Gross realized investment gains											68
Net realized investment gains (losses)											68
Other, including gains (losses) on calls and redemptions									(277)	(608)	69
Net realized gains (losses) on investments									$ (277)	$ (608)	$ 137