Note 7 - Segment Information - Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment operating income (loss)	$ 37,407	$ 21,572	$ 62,256	$ 29,753
Segment income (loss) before taxes	34,583	17,809	56,641	21,751
Segment assets	2,691,799	2,812,695	2,691,799	2,812,695	$ 2,603,047
Revenues from external customers	1,203,523	1,026,462	2,248,320	2,097,302
Truck Segment [Member]
Segment operating income (loss)	37,546	21,588	62,540	30,156
Segment income (loss) before taxes	34,766	17,865	57,010	22,235
Segment assets	2,655,860	2,779,789	2,655,860	2,779,789
Revenues from external customers	1,199,703	1,022,547	2,240,556	2,089,837
Other Segments [Member]
Segment operating income (loss)	(139)	(16)	(284)	(403)
Segment income (loss) before taxes	(183)	(56)	(368)	(484)
Segment assets	35,939	32,906	35,939	32,906
Revenues from external customers	$ 3,821	$ 3,915	$ 7,764	$ 7,465