Note 7 - Segment Information - Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenues from external customers	$ 1,376,136	$ 1,257,459	$ 3,965,748	$ 3,505,779
Segment operating income (loss)	58,649	48,436	134,464	110,692
Segment income (loss) before taxes	54,181	45,335	121,196	101,976
Segment assets	3,189,803	2,758,990	3,189,803	2,758,990	$ 2,890,139
Truck Segment [Member]
Revenues from external customers	1,371,472	1,253,429	3,952,396	3,493,984
Segment operating income (loss)	58,665	48,644	134,151	111,184
Segment income (loss) before taxes	54,250	45,590	121,037	102,599
Segment assets	3,152,204	2,721,463	3,152,204	2,721,463
Other Segments [Member]
Revenues from external customers	4,664	4,030	13,352	11,795
Segment operating income (loss)	(16)	(208)	313	(492)
Segment income (loss) before taxes	(69)	(255)	159	(623)
Segment assets	$ 37,599	$ 37,527	$ 37,599	$ 37,527