Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
OPERATING REVENUE	$ 645,262	$ 596,815	$ 610,803
Operating Expenses
Salaries, wages and benefits	269,482	240,139	227,872
Rent and purchased transportation	4,643	7,984	18,700
Fuel	86,094	101,871	110,536
Operations and maintenance	27,647	24,479	27,143
Operating taxes and licenses	14,962	14,459	16,390
Insurance and claims	22,229	17,003	17,227
Communications and utilities	5,281	4,953	6,086
Depreciation and amortization	109,937	100,212	100,519
Other operating expenses	26,398	22,781	21,506
Gain on disposal of property and equipment	(14,830)	(31,341)	(24,963)
Total operating expenses	551,843	502,540	521,016
Operating income	93,419	94,275	89,787
Interest income	842	3,955	2,130
Interest expense	0	(1,052)	0
Income before income taxes	94,261	97,178	91,917
Federal and state income tax expense	23,455	24,211	19,240
Net income	70,806	72,967	72,677
Other comprehensive income, net of tax	0	0	0
Comprehensive income	$ 70,806	$ 72,967	$ 72,677
Net income per share
Basic	$ 0.87	$ 0.89	$ 0.88
Diluted	$ 0.87	$ 0.89	$ 0.88
Weighted average shares outstanding
Basic	81,388	81,980	82,378
Diluted	81,444	82,024	82,410
Dividends declared per share	$ 0.08	$ 0.08	$ 0.08