Note 16 - Segment Information - Segment 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	[2]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues		$ 203,306	[1]	$ 178,631	$ 164,326	$ 158,744	$ 190,978	$ 164,500	$ 158,832	$ 156,341		$ 705,007	$ 670,651	$ 724,240
Operating income (loss)		14,843	[1]	$ 9,041	$ 3,962	$ 309	12,267	$ 5,446	$ 7,316	$ 7,418		28,155	32,447	67,782
Total assets		649,668					620,538					649,668	620,538
Operating Segments [Member]
Revenues												711,016	674,828	726,975
Operating income (loss)												28,155	32,447	67,782
Depreciation and amortization (1)	[3]											76,447	72,456	61,384
Total assets		649,668					620,538					649,668	620,538	646,717
Capital expenditures, net (2)	[4]											72,006	59,052	148,994
Operating Segments [Member] | Truckload [Member]
Revenues												612,834	601,226	655,918
Operating income (loss)												38,781	37,031	74,107
Depreciation and amortization (1)	[3]											75,013	71,173	60,138
Total assets		557,399					548,882					557,399	548,882	580,506
Capital expenditures, net (2)	[4]											70,300	57,242	147,896
Operating Segments [Member] | Other Segments [Member]
Revenues												98,182	73,602	71,057
Operating income (loss)												8,588	7,631	5,768
Depreciation and amortization (1)	[3]											24	22	13
Total assets		42,479					31,289					42,479	31,289	26,315
Capital expenditures, net (2)	[4]											810	43	29
Operating Segments [Member] | Corporate Segment [Member]
Revenues
Operating income (loss)												(19,214)	(12,215)	(12,093)
Depreciation and amortization (1)	[3]											1,410	1,261	1,233
Total assets		$ 49,790					$ 40,367					49,790	40,367	39,896
Capital expenditures, net (2)	[4]											896	1,767	1,069
Intersegment Eliminations [Member]
Revenues												(6,009)	(4,177)	(2,735)
Intersegment Eliminations [Member] | Truckload [Member]
Revenues
Intersegment Eliminations [Member] | Other Segments [Member]
Revenues												(6,009)	(4,177)	(2,735)
Intersegment Eliminations [Member] | Corporate Segment [Member]
Revenues

[1] Includes $40.1m one-time benefit related to the Tax Cuts and Jobs Act.

[2] Adjusted from 10-Q as filed due to implementation of ASU 2016-09.

[3] Includes gains and losses on disposition of equipment.

[4] Includes equipment purchased under capital leases.