Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating revenues	$ 2,372,178	$ 2,463,701	$ 2,457,914
Operating expenses:
Salaries, wages and benefits	795,847	818,487	781,064
Fuel	157,124	235,928	254,564
Supplies and maintenance	175,842	182,909	185,074
Taxes and licenses	95,746	95,525	87,318
Insurance and claims	109,816	88,913	98,133
Depreciation	263,286	249,527	230,151
Rent and purchased transportation	519,184	549,438	589,002
Communications and utilities	14,474	15,303	16,063
Other	13,421	2,199	(7,670)
Total operating expenses	2,144,740	2,238,229	2,233,699
Operating income	227,438	225,472	224,215
Other expense (income):
Interest expense	4,215	6,854	2,695
Interest income	(1,634)	(3,326)	(2,737)
Other	163	38	376
Total other expense (income)	2,744	3,566	334
Income before income taxes	224,694	221,906	223,881
Income tax expense	55,616	54,962	55,733
Net income	$ 169,078	$ 166,944	$ 168,148
Earnings per share:
Basic	$ 2.45	$ 2.40	$ 2.35
Diluted	$ 2.44	$ 2.38	$ 2.33
Weighted-average common shares outstanding:
Basic	69,018	69,567	71,694
Diluted	69,427	70,026	72,057