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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Marketable Securities [Line Items]
Proceeds from sales									$ 55,090	$ 15,028	$ 27,764
Gross realized investment gains									176	239	486
Gross realized investment (losses)									(68)	(1,354)	(5)
Net realized investment gains (losses)									108	(1,115)	481
Other, including gains (losses) on calls and redemptions									(188)	(421)	(1,128)
Net realized gains (losses) on investments	$ (53)	$ (4)	$ (82)	$ 59	$ (65)	$ (329)	$ (1,111)	$ (31)	(80)	(1,536)	(647)
Fixed Maturity Securities Available for Sale [Member]
Marketable Securities [Line Items]
Proceeds from sales									33,113	15,025	14,591
Gross realized investment gains									140	238	485
Gross realized investment (losses)									(68)	(1,354)	(5)
Net realized investment gains (losses)									72	(1,116)	480
Other, including gains (losses) on calls and redemptions									(196)	(144)	(520)
Net realized gains (losses) on investments									(124)	(1,260)	(40)
Equity Securities [Member]
Marketable Securities [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]
Marketable Securities [Line Items]
Proceeds from sales									21,977		13,172
Gross realized investment gains									36
Net realized investment gains (losses)									36
Other, including gains (losses) on calls and redemptions									8	(277)	(608)
Net realized gains (losses) on investments									$ 44	$ (277)	$ (608)