Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Operating Revenue	$ 428,732	$ 364,404	$ 821,537	$ 687,494
Operating Expenses:
Salaries, wages and employees’ benefits	220,406	196,388	431,530	377,291
Purchased transportation	34,113	28,639	64,029	49,459
Fuel, operating expenses and supplies	84,745	66,092	163,539	130,082
Operating taxes and licenses	12,794	10,875	24,944	21,457
Claims and insurance	9,910	10,426	20,101	19,475
Depreciation and amortization	25,241	22,182	48,271	42,269
Loss (gain) from property disposals, net	(42)	116	(21)	248
Total operating expenses	387,167	334,718	752,393	640,281
Operating Income	41,565	29,686	69,144	47,213
Nonoperating Expenses (Income):
Interest expense	1,454	1,538	2,680	2,449
Other, net	(142)	90	(245)	188
Nonoperating expenses, net	1,312	1,628	2,435	2,637
Income Before Income Taxes	40,253	28,058	66,709	44,576
Income Tax Provision	9,972	10,487	15,303	15,610
Net Income	$ 30,281	$ 17,571	$ 51,406	$ 28,966
Weighted average common shares outstanding – basic	25,766	25,501	25,732	25,477
Weighted average common shares outstanding – diluted	26,354	26,000	26,326	25,982
Basic Earnings Per Share	$ 1.18	$ 0.69	$ 2.00	$ 1.14
Diluted Earnings Per Share	$ 1.15	$ 0.68	$ 1.95	$ 1.12