Consolidated Statements of Income and Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Revenue	$ 1,315,311		$ 3,788,406
Lease and rental	60,825	$ 54,630	177,342	$ 158,922
Total revenue	1,376,136	1,257,459	3,965,748	3,505,779
Cost of products sold:
Lease and rental	49,924	45,197	147,015	133,707
Total cost of products sold	1,127,079	1,037,411	3,246,319	2,888,752
Gross profit	249,057	220,048	719,429	617,027
Selling, general and administrative expense	177,405	159,281	527,729	469,037
Depreciation and amortization expense	12,794	12,438	57,395	37,374
(Loss) gain on sale of assets	(209)	107	159	76
Operating income	58,649	48,436	134,464	110,692
Interest expense, net	4,468	3,101	13,268	8,716
Income before taxes	54,181	45,335	121,196	101,976
Provision for income taxes	12,516	15,551	29,103	35,714
Net income	$ 41,665	$ 29,784	$ 92,093	$ 66,262
Earnings per common share:
Basic (in dollars per share)	$ 1.06	$ 0.75	$ 2.33	$ 1.68
Diluted (in dollars per share)	$ 1.03	$ 0.72	$ 2.27	$ 1.62
Weighted average shares outstanding:
Basic (in shares)	39,309	39,825	39,480	39,560
Diluted (in shares)	40,388	41,146	40,635	40,830
Dividends declared per common share (in dollars per share)	$ 0.12		$ 0.12
Comprehensive income	$ 41,665	$ 29,784	$ 92,093	$ 66,548
New and Used Commercial Vehicle [Member]
Revenues:
Revenue	878,845	819,028	2,508,970	2,230,969
Cost of products sold:
Cost of products sold	808,634	754,762	2,311,156	2,061,135
Aftermarket Products and Services [Member]
Revenues:
Revenue	426,845	375,835	1,250,080	1,092,540
Cost of products sold:
Cost of products sold	268,521	237,452	788,148	693,910
Finance and Insurance [Member]
Revenues:
Revenue	5,053	4,771	15,286	13,092
Product and Service, Other [Member]
Revenues:
Revenue	$ 4,568	$ 3,195	$ 14,070	$ 10,256