Note 17 - Segments - Segment Reporting Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from external customers	$ 1,317,704	$ 1,599,265	$ 1,544,561	$ 1,348,317	$ 1,540,442	$ 1,376,136	$ 1,348,831	$ 1,240,781	$ 5,809,847	$ 5,506,190	$ 4,713,882
Interest income									1,680	1,376	891
Interest expense									30,487	21,058	13,201
Depreciation and amortization									55,372	70,489	50,069
Segment operating income (loss)	39,423	58,323	61,792	56,867	68,387	58,649	43,426	32,389	216,405	202,851	148,709
Segment income from continuing operations before taxes	33,345	$ 52,210	$ 54,410	$ 49,558	61,973	$ 54,181	$ 38,932	$ 28,083	189,523	183,169	136,399
Segment assets	3,407,329				3,201,350				3,407,329	3,201,350	2,890,139
Goodwill	292,142				291,391				292,142	291,391	291,391
Expenditures for segment assets									293,493	238,260	209,917
Truck Segment [Member]
Revenues from external customers									5,794,155	5,488,787	4,698,035
Interest income									1,680	1,376	891
Interest expense									30,201	20,850	13,024
Depreciation and amortization									55,036	70,170	49,634
Segment operating income (loss)									216,691	202,725	149,338
Segment income from continuing operations before taxes									188,122	183,251	137,205
Segment assets	3,369,517				3,166,174				3,369,517	3,166,174	2,855,001
Goodwill	289,582				288,831				289,582	288,831	288,831
Expenditures for segment assets									292,980	238,229	209,852
Other Segments [Member]
Revenues from external customers									15,692	17,403	15,847
Interest income									0	0	0
Interest expense									286	208	177
Depreciation and amortization									336	319	435
Segment operating income (loss)									(286)	126	(629)
Segment income from continuing operations before taxes									1,401	(82)	(806)
Segment assets	37,812				35,176				37,812	35,176	35,138
Goodwill	$ 2,560				$ 2,560				2,560	2,560	2,560
Expenditures for segment assets									$ 513	$ 31	$ 65