Consolidated Statements of Earnings - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating revenues	$ 9,165,258	$ 8,614,874	$ 7,189,568
Operating expenses:
Rents and purchased transportation	4,528,812	4,434,540	3,650,806
Salaries, wages and employee benefits	2,167,851	1,926,213	1,608,378
Depreciation and amortization	499,145	435,893	383,518
Fuel and fuel taxes	463,195	459,011	347,573
Operating supplies and expenses	333,113	303,529	257,239
General and administrative expenses, net of asset dispositions	191,933	163,270	125,878
Insurance and claims	157,251	129,406	123,579
Operating taxes and licenses	55,336	51,080	44,825
Communication and utilities	34,797	30,911	23,983
Total operating expenses	8,431,433	7,933,853	6,565,779
Operating income	733,825	681,021	623,789
Interest income	1,754	224	235
Interest expense	54,684	40,427	28,785
Earnings before income taxes	680,895	640,818	595,239
Income taxes	164,575	151,233	(91,024)
Net earnings	$ 516,320	$ 489,585	$ 686,263
Weighted average basic shares outstanding (in shares)	107,329	109,375	109,987
Basic earnings per share (in dollars per share)	$ 4.81	$ 4.48	$ 6.24
Weighted average diluted shares outstanding (in shares)	108,307	110,428	111,049
Diluted earnings per share (in dollars per share)	$ 4.77	$ 4.43	$ 6.18
Dividends declared per common share (in dollars per share)	$ 1.04	$ 0.96	$ 0.92
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 8,122,600	$ 7,557,648	$ 6,435,858
Fuel Surcharge [Member]
Operating revenues	$ 1,042,658	$ 1,057,226	$ 753,710