Note 7 - Segment Information - Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Revenues from external customers	$ 1,240,781	$ 1,044,797
Segment operating income	32,389	24,849
Segment income (loss) before taxes	28,083	22,058
Segment assets	2,899,204	2,639,561	$ 2,890,139
Truck Segment [Member]
Revenues from external customers	1,236,652	1,040,853
Segment operating income	32,360	24,994
Segment income (loss) before taxes	28,103	22,244
Segment assets	2,864,504	2,603,945
Other Segments [Member]
Revenues from external customers	4,129	3,944
Segment operating income	29	(145)
Segment income (loss) before taxes	(20)	(186)
Segment assets	$ 34,700	$ 35,616