Summary of Significant Accounting Policies - Summary of Effect of Adopting ASC 606 In Consolidated Statement of Comprehensive Income (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
Net income	$ 5,375	$ 3,950	$ 7,788	$ (1,733)	$ 2,093	$ 3,953	$ 6,064	$ 3,030	$ 15,380	$ 15,140	$ 17,651
Comprehensive income									12,264	$ 20,701	$ 14,904
ASC 606 [Member] | Amounts as if Previous Guidance in Effect [Member]
Net income									13,982
Comprehensive income									$ 10,866