Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating revenue	$ 874,374	$ 843,271	$ 787,594
Operating expenses (income):
Salaries, wages and benefits	300,155	274,156	252,047
Purchased transportation	154,613	158,948	144,611
Fuel and fuel taxes	98,133	121,341	121,633
Supplies and maintenance	47,815	46,737	40,853
Depreciation	102,893	95,137	88,585
Operating taxes and licenses	10,840	10,100	9,473
Insurance and claims	47,454	38,201	38,657
Communications and utilities	8,011	7,709	6,634
Other	21,665	23,124	21,997
Total operating expenses	781,128	766,773	717,246
Operating income	93,246	76,498	70,348
Other	(140)	(1,190)	(681)
Income before income taxes	93,386	77,688	71,029
Income taxes expense	23,886	16,617	16,002
Net income	$ 69,500	$ 61,071	$ 55,027
Basic earnings per common share (in dollars per share)	$ 0.84	$ 0.75	$ 0.67
Diluted earnings per common share (in dollars per share)	0.84	0.74	0.67
Dividends declared per common share (in dollars per share)	$ 0.633	$ 0.513	$ 0.067
Revenue Equipment [Member]
Operating expenses (income):
Gain on disposition of property, plant and equipment	$ (8,733)	$ (8,680)	$ (7,244)
Facility [Member]
Operating expenses (income):
Gain on disposition of property, plant and equipment	$ (1,718)	$ 0	$ 0