Quarterly Financial Information - Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operations Data
Rental revenues	$ 88,517	$ 88,138	$ 85,629	$ 89,506	$ 92,231	$ 90,857	$ 88,105	$ 82,696	$ 351,790	$ 353,889	$ 318,774
Total revenues	148,980	156,448	137,673	129,453	147,221	173,562	127,439	122,008	572,554	570,230	498,330
Gross profit	70,654	68,204	63,232	61,656	70,997	78,282	59,881	57,005	263,746	266,165	233,251
Income from operations	41,026	37,333	32,692	29,702	38,248	46,748	29,066	27,310	140,753	141,372	117,481
Income before provision for income taxes	39,310	35,495	30,625	26,614	35,454	43,455	25,965	24,251	132,044	129,125	104,695
Net income	$ 31,175	$ 28,101	$ 22,549	$ 20,159	$ 26,401	$ 32,468	$ 19,488	$ 18,449	$ 101,984	$ 96,806	$ 79,406
Earnings per share:
Basic	$ 1.29	$ 1.17	$ 0.93	$ 0.83	$ 1.09	$ 1.34	$ 0.80	$ 0.76	$ 4.22	$ 3.99	$ 3.29
Diluted	1.27	1.15	0.92	0.81	1.07	1.32	0.79	0.75	4.16	3.93	3.24
Dividends declared per share	$ 0.420	$ 0.420	$ 0.420	$ 0.420	$ 0.375	$ 0.375	$ 0.375	$ 0.375	$ 1.68	$ 1.50	$ 1.36
Shares used in per share calculations:
Basic	24,119	24,097	24,121	24,292	24,290	24,268	24,246	24,195	24,157	24,250	24,141
Diluted	24,453	24,443	24,471	24,738	24,697	24,632	24,579	24,540	24,531	24,623	24,540
Balance Sheet Data
Rental equipment, net	$ 938,116	$ 948,125	$ 969,681	$ 975,709	$ 967,500	$ 958,610	$ 943,152	$ 912,878	$ 938,116	$ 967,500	$ 901,015
Total assets	1,275,744	1,295,380	1,312,342	1,314,429	1,309,875	1,306,223	1,280,249	1,239,633	1,275,744	1,309,875
Notes payable	222,754	249,980	272,149	291,544	293,431	301,469	301,878	289,464	222,754	293,431
Shareholders’ equity	$ 682,604	$ 661,614	$ 643,836	$ 637,604	$ 634,036	$ 616,715	$ 592,309	$ 580,643	$ 682,604	$ 634,036	$ 571,535	$ 524,184