Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Operating Revenue	$ 392,805	$ 323,090
Operating Expenses:
Salaries, wages and employees’ benefits	211,124	180,903
Purchased transportation	29,916	20,820
Fuel, operating expenses and supplies	78,794	63,990
Operating taxes and licenses	12,150	10,582
Claims and insurance	10,191	9,049
Depreciation and amortization	23,030	20,087
Loss from property disposals, net	21	132
Total operating expenses	365,226	305,563
Operating Income	27,579	17,527
Nonoperating Expenses (Income):
Interest expense	1,226	911
Other, net	(103)	98
Nonoperating expenses, net	1,123	1,009
Income Before Income Taxes	26,456	16,518
Income Tax Provision	5,331	5,123
Net Income	$ 21,125	$ 11,395
Weighted average common shares outstanding – basic	25,698	25,225
Weighted average common shares outstanding – diluted	26,308	25,971
Basic Earnings Per Share	$ 0.82	$ 0.45
Diluted Earnings Per Share	$ 0.80	$ 0.44