Note 7 - Segment Information - Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Revenues from external customers	$ 1,257,459	$ 1,096,041	$ 3,505,779	$ 3,193,343
Segment income (loss) before taxes	45,335	24,303	101,976	46,054
Segment assets	2,758,990	2,695,495	2,758,990	2,695,495	$ 2,603,047
Truck Segment [Member]
Revenues from external customers	1,253,429	1,092,005	3,493,984	3,181,842
Segment income (loss) before taxes	45,590	24,505	102,599	46,740
Segment assets	2,721,463	2,662,872	2,721,463	2,662,872
Other Segments [Member]
Revenues from external customers	4,030	4,036	11,795	11,501
Segment income (loss) before taxes	(255)	(202)	(623)	(686)
Segment assets	$ 37,527	$ 32,623	$ 37,527	$ 32,623