Quarterly Financial Data (Unaudited) - Condensed Summary of Actual Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 223,602	$ 225,010	$ 233,391	$ 215,734	$ 188,339	$ 198,349	$ 195,127	$ 173,416	$ 897,737	$ 755,231	$ 662,112
Gross Profit	30,267	32,177	35,585	30,823	22,658	25,052	22,603	21,405	128,852	91,718	86,703
Operating Income	14,445	16,243	20,909	15,276	8,020	10,693	7,578	4,560	66,873	30,851	24,916
Net Income (Loss)	$ 8,881	$ 12,583	$ 13,195	$ 9,853	$ (7,227)	$ 4,763	$ 131	$ 628	$ 44,512	$ (1,705)	$ 6,785
Basic (in dollars per share)	$ 0.29	$ 0.42	$ 0.44	$ 0.33	$ (0.24)	$ 0.16	$ 0.00	$ 0.02	$ 1.47	$ (0.06)	$ 0.23
Diluted (in dollars per share)	$ 0.29	$ 0.41	$ 0.43	$ 0.32	$ (0.24)	$ 0.16	$ 0.00	$ 0.02	$ 1.46	$ (0.06)	$ 0.23