Quarterly Financial Information - Quarterly Financial Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operations Data
Rental revenues	$ 71,694	$ 69,642	$ 65,809	$ 62,430	$ 66,181	$ 65,941	$ 63,043	$ 60,601	$ 269,575	$ 255,766	$ 248,444
Total revenues	111,792	113,025	95,745	87,560	94,772	108,839	87,156	88,742	408,122	379,509	364,039
Gross profit	52,204	49,298	42,079	38,638	44,630	45,189	40,169	39,027	182,219	169,015	168,259
Income from operations	26,796	25,098	18,239	15,227	21,678	22,806	18,377	17,389	85,360	80,250	81,981
Income before provision for income taxes	24,222	22,609	16,794	12,936	19,341	20,679	16,197	15,157	76,561	71,374	72,867
Net income	13,887	13,746	10,205	7,871	11,761	12,573	9,848	9,215	45,709	43,397	44,777
Earnings per share:
Basic	$ 0.54	$ 0.53	$ 0.39	$ 0.31	$ 0.46	$ 0.49	$ 0.39	$ 0.37	$ 1.77	$ 1.71	$ 1.80
Diluted	$ 0.53	$ 0.53	$ 0.39	$ 0.30	$ 0.45	$ 0.48	$ 0.38	$ 0.36	$ 1.75	$ 1.67	$ 1.78
Dividends declared per share	$ 0.245	$ 0.245	$ 0.245	$ 0.245	$ 0.24	$ 0.24	$ 0.24	$ 0.24	$ 0.98	$ 0.96	$ 0.94
Shares used in per share calculations:
Basic	25,999	25,953	25,912	25,789	25,717	25,649	25,354	25,003	25,914	25,433	24,759
Diluted	26,206	26,152	26,220	26,230	26,211	26,095	25,818	25,435	26,175	25,926	25,156
Balance Sheet Data
Rental equipment, net	825,750	817,898	795,532	775,875	767,010	753,810	737,636	726,100	825,750	767,010	718,853
Total assets	1,116,407	1,089,130	1,051,764	1,021,613	1,027,611	1,009,435	985,262	967,131	1,116,407	1,027,611
Notes payable	322,478	322,280	307,000	288,081	290,003	280,902	278,875	281,251	322,478	290,003
Shareholders' equity	$ 424,531	$ 414,519	$ 406,439	$ 401,563	$ 401,030	$ 395,694	$ 387,779	$ 372,864	$ 424,531	$ 401,030	$ 364,738	$ 333,142