Quarterly Financial Information - Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operations Data
Rental revenues	$ 77,705	$ 73,781	$ 69,953	$ 67,978	$ 70,352	$ 67,757	$ 66,747	$ 66,532	$ 289,417	$ 271,388	$ 273,696
Total revenues	122,227	135,388	109,582	94,837	105,283	121,993	103,105	93,699	462,034	424,080	404,544
Gross profit	54,689	58,775	49,211	43,670	49,323	50,433	43,756	40,655	206,345	184,167	176,907
Income from operations	26,786	30,286	21,846	15,822	22,696	24,232	18,073	14,258	94,740	79,259	76,957
Income before (benefit) provision for income taxes	23,949	27,336	18,908	13,259	19,795	21,277	15,006	10,853	83,452	66,931	66,377
Net income	$ 117,724	$ 16,762	$ 11,461	$ 7,973	$ 9,734	$ 12,872	$ 9,079	$ 6,566	$ 153,920	$ 38,251	$ 40,470
Earnings per share:
Basic	$ 4.90	$ 0.70	$ 0.48	$ 0.33	$ 0.41	$ 0.54	$ 0.38	$ 0.28	$ 6.41	$ 1.60	$ 1.60
Diluted	4.82	0.69	0.48	0.33	0.40	0.54	0.38	0.27	6.34	1.60	1.59
Dividends declared per share	$ 0.260	$ 0.260	$ 0.260	$ 0.260	$ 0.255	$ 0.255	$ 0.255	$ 0.255	$ 1.04	$ 1.02	$ 1.00
Shares used in per share calculations:
Basic	24,044	24,015	23,985	23,950	23,927	23,911	23,900	23,862	23,999	23,900	25,369
Diluted	24,410	24,228	24,092	24,232	24,123	24,041	23,949	23,911	24,269	23,976	25,457
Balance Sheet Data
Rental equipment, net	$ 862,320	$ 865,724	$ 863,207	$ 856,981	$ 856,371	$ 862,528	$ 868,422	$ 867,215	$ 862,320	$ 856,371	$ 869,601
Total assets	1,147,854	1,156,415	1,150,123	1,130,734	1,128,276	1,144,923	1,148,018	1,132,355	1,147,854	1,128,276
Notes payable	303,414	323,117	330,287	323,483	326,266	345,286	363,121	365,772	303,414	326,266
Shareholders’ equity	$ 524,184	$ 412,782	$ 402,365	$ 396,625	$ 394,287	$ 390,600	$ 383,313	$ 380,512	$ 524,184	$ 394,287	$ 379,687	$ 424,531