Investments - Summary of Net Investment Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Gross investment income (loss)	$ 15,394	$ 13,447	$ 10,911
Investment expenses	2,114	2,115	1,730
Net investment income, less investment expenses	13,280	11,332	9,181
Debt Securities Available-for-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross investment income (loss)	9,591	10,368	8,709
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross investment income (loss)	1,333	1,922	1,955
Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross investment income (loss)	1,703	960	226
Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross investment income (loss)	$ 2,767	$ 197	$ 21,000