Quarterly Results (Unaudited) - Summary of Unaudited Quarterly Results of Operations (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 143,573,000	$ 140,734,000	$ 136,431,000	$ 137,612,000	$ 133,605,000	$ 128,403,000	$ 128,219,000	$ 128,459,000	$ 558,350,000	$ 518,686,000	$ 503,584,000
Gross Profit	27,149,000	25,569,000	25,752,000	28,783,000	27,721,000	25,577,000	25,912,000	25,582,000	107,253,000	104,792,000	104,544,000
Net income	$ 8,958,000	$ 9,569,000	$ 8,393,000	$ 8,775,000	$ 7,987,000	$ 7,049,000	$ 6,952,000	$ 7,491,000	$ 35,695,000	$ 29,479,000	$ 28,525,000
Net Income per Basic Common Share	$ 0.47	$ 0.51	$ 0.45	$ 0.47	$ 0.42	$ 0.36	$ 0.36	$ 0.38	$ 1.90	$ 1.52	$ 1.44
Net Income per Diluted Common Share	$ 0.47	$ 0.50	$ 0.44	$ 0.46	$ 0.42	$ 0.36	$ 0.35	$ 0.38	$ 1.87	$ 1.51	$ 1.43