Quarterly Financial Information - Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operations Data
Rental revenues	$ 70,694	$ 70,195	$ 67,305	$ 65,502	$ 71,694	$ 69,642	$ 65,809	$ 62,430	$ 273,696	$ 269,575	$ 255,766
Total revenues	105,282	113,048	96,026	90,188	111,792	113,025	95,745	87,560	404,544	408,122	379,509
Gross profit	46,756	50,146	40,918	39,087	52,204	49,298	42,079	38,638	176,907	182,219	169,015
Income from operations	21,467	25,150	16,465	13,875	26,796	25,098	18,239	15,227	76,957	85,360	80,250
Income before provision for income taxes	18,523	22,505	14,033	11,316	24,222	22,609	16,794	12,936	66,377	76,561	71,374
Net income	$ 11,518	$ 13,616	$ 8,490	$ 6,846	$ 13,887	$ 13,746	$ 10,205	$ 7,871	$ 40,470	$ 45,709	$ 43,397
Earnings per share:
Basic	$ 0.48	$ 0.54	$ 0.33	$ 0.26	$ 0.54	$ 0.53	$ 0.39	$ 0.31	$ 1.60	$ 1.77	$ 1.71
Diluted	0.48	0.54	0.32	0.26	0.53	0.53	0.39	0.30	1.59	1.75	1.67
Dividends declared per share	$ 0.250	$ 0.250	$ 0.250	$ 0.250	$ 0.245	$ 0.245	$ 0.245	$ 0.245	$ 1.00	$ 0.98	$ 0.96
Shares used in per share calculations:
Basic	23,932	25,334	26,142	26,091	25,999	25,953	25,912	25,789	25,369	25,914	25,433
Diluted	24,015	25,408	26,273	26,276	26,206	26,152	26,220	26,230	25,457	26,175	25,926
Balance Sheet Data
Rental equipment, net	$ 869,601	$ 864,277	$ 856,489	$ 839,078	$ 825,750	$ 817,898	$ 795,532	$ 775,875	$ 869,601	$ 825,750	$ 767,010
Total assets	1,152,709	1,149,095	1,132,750	1,112,056	1,116,407	1,089,130	1,051,764	1,021,613	1,152,709	1,116,407
Notes payable	381,441	382,113	337,177	320,923	322,478	322,280	307,000	288,081	381,441	322,478
Shareholders' equity	$ 379,687	$ 389,464	$ 426,823	$ 425,848	$ 424,531	$ 414,519	$ 406,439	$ 401,563	$ 379,687	$ 424,531	$ 401,030	$ 364,738