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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment [Line Items]
Proceeds from sales									$ 59,339,000	$ 4,432,000	$ 10,548,000
Gross realized investment gains									891,000	177,000	750,000
Gross realized investment losses									(1,728,000)	(162,000)	(451,000)
Net realized investment gains (losses)									(837,000)	15,000	299,000
Impairments									0	(2,653,000)	(222,000)
Other, including gains on calls and redemptions									343,000	144,000	620,000
Net realized gains (losses) on investments	$ (1,468,000)	$ 181,000	$ 545,000	$ 248,000	$ 24,000	$ 40,000	$ (2,617,000)	$ 59,000	(494,000)	(2,494,000)	697,000
Fixed Maturity Securities Available for Sale [Member]
Investment [Line Items]
Proceeds from sales									54,730,000	4,432,000	768,000
Gross realized investment gains									823,000	177,000	1,000
Gross realized investment losses									(1,728,000)	(162,000)
Net realized investment gains (losses)									(905,000)	15,000	1,000
Impairments										(2,653,000)	(222,000)
Other, including gains on calls and redemptions									274,000	102,000	244,000
Net realized gains (losses) on investments									(631,000)	(2,536,000)	23,000
Equity Securities [Member]
Investment [Line Items]
Proceeds from sales											9,780,000
Gross realized investment gains											749,000
Gross realized investment losses											(451,000)
Net realized investment gains (losses)											298,000
Net realized gains (losses) on investments											298,000
Other [Member]
Investment [Line Items]
Proceeds from sales									4,609,000
Gross realized investment gains									68,000
Net realized investment gains (losses)									68,000
Other, including gains on calls and redemptions									69,000	42,000	376,000
Net realized gains (losses) on investments									$ 137,000	$ 42,000	$ 376,000