Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating revenues	$ 2,618,149	$ 2,280,826
Operating expenses:
Rents and purchased transportation	1,352,301	1,136,151
Salaries, wages and employee benefits	620,032	574,249
Depreciation and amortization	137,545	130,095
Fuel and fuel taxes	113,040	101,123
Operating supplies and expenses	81,698	85,598
General and administrative expenses, net of asset dispositions	44,891	45,165
Insurance and claims	38,030	32,361
Operating taxes and licenses	13,814	13,312
Communication and utilities	9,146	8,031
Total operating expenses	2,410,497	2,126,085
Operating income	207,652	154,741
Net interest expense	12,024	12,036
Earnings before income taxes	195,628	142,705
Income taxes	49,022	37,871
Net earnings	$ 146,606	$ 104,834
Weighted average basic shares outstanding (in shares)	105,678	105,985
Basic earnings per share (in dollars per share)	$ 1.39	$ 0.99
Weighted average diluted shares outstanding (in shares)	106,816	106,950
Diluted earnings per share (in dollars per share)	$ 1.37	$ 0.98
Dividends declared per common share (in dollars per share)	$ 0.28	$ 0.27
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 2,388,034	$ 2,045,694
Fuel Surcharge [Member]
Operating revenues	$ 230,115	$ 235,132