Investments - A Summary of the Company's Realized Gains and Losses on Sales, Calls or Redemptions of Investments (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Proceeds from sales									$ 27,764,000	$ 59,339,000	$ 4,432,000
Gross realized investment gains									486,000	891,000	177,000
Gross realized investment (losses)									(5,000)	(1,728,000)	(162,000)
Net realized investment gains (losses)									481,000	(837,000)	15,000
Impairments									0		(2,653,000)
Other, including gains (losses) on calls and redemptions									(1,128,000)	343,000	144,000
Net realized gains (losses) on investments	$ 114,000	$ (192,000)	$ (388,000)	$ (181,000)	$ (1,468,000)	$ 181,000	$ 545,000	$ 248,000	(647,000)	(494,000)	(2,494,000)
Fixed Maturity Securities Available for Sale [Member]
Investment [Line Items]
Proceeds from sales									14,591,000	54,730,000	4,432,000
Gross realized investment gains									485,000	823,000	177,000
Gross realized investment (losses)									(5,000)	(1,728,000)	(162,000)
Net realized investment gains (losses)									480,000	(905,000)	15,000
Impairments											(2,653,000)
Other, including gains (losses) on calls and redemptions									(520,000)	274,000	102,000
Net realized gains (losses) on investments									(40,000)	(631,000)	(2,536,000)
Equity Securities [Member]
Investment [Line Items]
Proceeds from sales									1,000
Gross realized investment gains									1,000
Net realized investment gains (losses)									1,000
Net realized gains (losses) on investments									1,000
Other [Member]
Investment [Line Items]
Proceeds from sales									13,172,000	4,609,000
Gross realized investment gains										68,000
Net realized investment gains (losses)										68,000
Other, including gains (losses) on calls and redemptions									(608,000)	69,000	42,000
Net realized gains (losses) on investments									$ (608,000)	$ 137,000	$ 42,000