Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Operating Revenue	$ 446,396	$ 410,584
Operating Expenses:
Salaries, wages and employees' benefits	238,645	220,352
Purchased transportation	30,059	28,418
Fuel, operating expenses and supplies	82,899	83,543
Operating taxes and licenses	14,396	13,202
Claims and insurance	10,421	9,530
Depreciation and amortization	32,590	26,782
Loss (gain) from property disposals, net	(1,390)	126
Total operating expenses	407,620	381,953
Operating Income	38,776	28,631
Nonoperating Expenses (Income):
Interest expense	1,402	1,383
Other, net	547	(334)
Nonoperating expenses, net	1,949	1,049
Income Before Income Taxes	36,827	27,582
Income Tax Provision	8,716	5,323
Net Income	$ 28,111	$ 22,259
Weighted average common shares outstanding – basic	26,070	25,873
Weighted average common shares outstanding – diluted	26,492	26,322
Basic Earnings Per Share	$ 1.08	$ 0.86
Diluted Earnings Per Share	$ 1.06	$ 0.85