Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Operating Revenue	$ 484,074	$ 446,396
Operating Expenses:
Salaries, wages and employees' benefits	244,437	238,645
Purchased transportation	45,031	30,059
Fuel, operating expenses and supplies	84,901	82,899
Operating taxes and licenses	14,338	14,396
Claims and insurance	11,480	10,421
Depreciation and amortization	35,372	32,590
Gain from property disposals, net	(199)	(1,390)
Total operating expenses	435,360	407,620
Operating Income	48,714	38,776
Nonoperating Expenses (Income):
Interest expense	852	1,402
Other, net	(131)	547
Nonoperating expenses, net	721	1,949
Income Before Income Taxes	47,993	36,827
Income Tax Provision	10,702	8,716
Net Income	$ 37,291	$ 28,111
Weighted average common shares outstanding – basic	26,285	26,070
Weighted average common shares outstanding – diluted	26,671	26,492
Basic Earnings Per Share	$ 1.42	$ 1.08
Diluted Earnings Per Share	$ 1.40	$ 1.06