Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating revenue	$ 973,644	$ 874,374	$ 843,271
Operating expenses (income):
Salaries, wages and benefits	317,962	300,155	274,156
Purchased transportation	193,221	154,613	158,948
Fuel and fuel taxes	131,288	98,133	121,341
Supplies and maintenance	45,459	47,815	46,737
Depreciation	102,562	102,893	95,137
Operating taxes and licenses	10,534	10,840	10,100
Insurance and claims	41,987	47,454	38,201
Communications and utilities	8,350	8,011	7,709
Other	26,887	21,665	23,124
Total operating expenses	861,955	781,128	766,773
Operating income	111,689	93,246	76,498
Other	(43)	(140)	(1,190)
Income before income taxes	111,732	93,386	77,688
Income taxes expense	26,304	23,886	16,617
Net income	$ 85,428	$ 69,500	$ 61,071
Basic earnings per common share (in dollars per share)	$ 1.03	$ 0.84	$ 0.75
Diluted earnings per common share (in dollars per share)	1.02	0.84	0.74
Dividends declared per common share (in dollars per share)	$ 0.66	$ 0.633	$ 0.513
Revenue Equipment [Member]
Operating expenses (income):
Gain on disposition of property, plant and equipment	$ (16,295)	$ (8,733)	$ (8,680)
Facility [Member]
Operating expenses (income):
Gain on disposition of property, plant and equipment	$ 0	$ (1,718)	$ 0