Note 18 - Segments - Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from external customers	$ 1,208,103	$ 1,257,459	$ 1,203,523	$ 1,044,797	$ 1,021,271	$ 1,096,041	$ 1,026,462	$ 1,070,840	$ 4,713,882	$ 4,214,614	$ 4,979,733
Interest income									891	621	490
Interest expense									13,201	14,900	13,963
Depreciation and amortization									50,069	51,261	43,859
Segment operating income	38,017	48,436	37,407	24,849	23,387	27,588	21,572	8,181	148,709	80,728	121,276
Segment income (loss) from continuing operations before taxes	34,423	$ 45,335	$ 34,583	$ 22,058	20,395	$ 24,303	$ 17,809	$ 3,942	136,399	66,449	107,803
Segment assets	2,890,139				2,603,047				2,890,139	2,603,047	2,852,008
Goodwill	291,391				290,191				291,391	290,191	285,041
Expenditures for segment assets									209,917	196,965	367,790
Truck Segment [Member]
Revenues from external customers									4,698,035	4,199,348	4,964,642
Interest income									891	621	490
Interest expense									13,024	14,740	13,814
Depreciation and amortization									49,634	50,771	43,355
Segment operating income									149,338	81,483	122,907
Segment income (loss) from continuing operations before taxes									137,205	67,364	109,583
Segment assets	2,855,001				2,570,016				2,855,001	2,570,016	2,818,255
Goodwill	288,831				287,631				288,831	287,631	282,481
Expenditures for segment assets									209,852	196,704	367,482
Other Segments [Member]
Revenues from external customers									15,847	15,266	15,091
Interest income
Interest expense									177	160	149
Depreciation and amortization									435	490	504
Segment operating income									(629)	(756)	(1,631)
Segment income (loss) from continuing operations before taxes									(806)	(915)	(1,780)
Segment assets	35,138				33,031				35,138	33,031	33,753
Goodwill	$ 2,560				$ 2,560				2,560	2,560	2,560
Expenditures for segment assets									$ 65	$ 261	$ 308