Quarterly Financial Information (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Operations Data
Rental Revenues	$ 65,117	$ 63,418	$ 60,389	$ 59,520	$ 62,798	$ 60,964	$ 57,118	$ 54,026	$ 248,444	$ 234,906	$ 200,615
Total Revenues	101,950	99,430	83,765	78,929	85,206	104,961	79,530	72,973	364,074	342,670	291,400
Gross Profit	44,039	43,640	40,780	39,835	44,193	47,387	39,359	35,852	168,294	166,791	130,815
Income from Operations	21,133	22,792	19,617	18,474	23,350	27,306	20,714	17,294	82,016	88,664	65,236
Income Before Provision for Income Taxes	18,851	20,480	17,235	16,301	21,231	25,255	18,760	15,812	72,867	81,058	59,050
Net Income	11,936	12,451	10,479	9,911	13,227	15,355	11,406	9,614	44,777	49,602	36,479
Earnings Per Share:
Basic	$ 0.48	$ 0.50	$ 0.42	$ 0.40	$ 0.54	$ 0.63	$ 0.47	$ 0.40	$ 1.80	$ 2.04	$ 1.52
Diluted	$ 0.47	$ 0.50	$ 0.42	$ 0.39	$ 0.53	$ 0.62	$ 0.46	$ 0.39	$ 1.78	$ 2.00	$ 1.50
Dividends Declared Per Share	$ 0.235	$ 0.235	$ 0.235	$ 0.235	$ 0.23	$ 0.23	$ 0.23	$ 0.23	$ 0.94	$ 0.92	$ 0.90
Shares Used in Per Share Calculations:
Basic	24,847	24,785	24,765	24,639	24,431	24,362	24,340	24,258	24,759	24,349	23,944
Diluted	25,216	25,106	25,149	25,183	24,892	24,719	24,742	24,660	25,156	24,760	24,289
Balance Sheet Data
Rental Equipment, net	718,853	719,636	710,918	693,577	673,146	655,854	632,518	608,115	718,853	673,146
Total Assets	972,446	988,254	963,649	934,282	918,929	895,144	866,700	832,032	972,446	918,929
Notes Payable	302,000	314,193	308,000	292,118	296,500	289,573	270,500	265,771	302,000	296,500
Shareholders' Equity	$ 364,738	$ 355,758	$ 347,500	$ 341,853	$ 333,142	$ 320,410	$ 309,322	$ 300,976	$ 364,738	$ 333,142	$ 294,977	$ 267,413