Quarterly Financial Information - Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operations Data
Rental revenues	$ 85,091	$ 82,155	$ 77,267	$ 74,261	$ 77,705	$ 73,781	$ 69,953	$ 67,978	$ 318,774	$ 289,417	$ 271,388
Total revenues	133,115	143,147	116,983	105,085	122,227	135,388	109,582	94,837	498,330	462,034	424,080
Gross profit	65,103	64,050	53,928	50,170	54,689	58,775	49,211	43,670	233,251	206,345	184,167
Income from operations	35,166	35,824	24,449	22,042	26,786	30,286	21,846	15,822	117,481	94,740	79,259
Income before provision (benefit) for income taxes	32,018	32,553	21,106	19,018	23,949	27,336	18,908	13,259	104,695	83,452	66,931
Net income	$ 24,249	$ 24,779	$ 15,912	$ 14,466	$ 117,724	$ 16,762	$ 11,461	$ 7,973	$ 79,406	$ 153,920	$ 38,251
Earnings per share:
Basic	$ 1.00	$ 1.03	$ 0.66	$ 0.60	$ 4.90	$ 0.70	$ 0.48	$ 0.33	$ 3.29	$ 6.41	$ 1.60
Diluted	0.99	1.01	0.65	0.59	4.82	0.69	0.48	0.33	3.24	6.34	1.60
Dividends declared per share	$ 0.34	$ 0.34	$ 0.34	$ 0.34	$ 0.26	$ 0.26	$ 0.26	$ 0.26	$ 1.36	$ 1.04	$ 1.02
Shares used in per share calculations:
Basic	24,179	24,172	24,145	24,067	24,044	24,015	23,985	23,950	24,141	23,999	23,900
Diluted	24,514	24,563	24,584	24,478	24,410	24,228	24,092	24,232	24,540	24,269	23,976
Balance Sheet Data
Rental equipment, net	$ 901,015	$ 888,607	$ 876,522	$ 865,338	$ 862,320	$ 865,724	$ 863,207	$ 856,981	$ 901,015	$ 862,320	$ 856,371
Total assets	1,217,316	1,201,799	1,180,209	1,148,858	1,147,854	1,156,415	1,150,123	1,130,734	1,217,316	1,147,854
Notes payable	298,564	309,006	314,860	300,595	303,414	323,117	330,287	323,483	298,564	303,414
Shareholders’ equity	$ 571,535	$ 554,547	$ 537,195	$ 530,284	$ 524,184	$ 412,782	$ 402,365	$ 396,625	$ 571,535	$ 524,184	$ 394,287	$ 379,687