Consolidated Statements of Earnings - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating revenues	$ 8,614,874	$ 7,189,568	$ 6,555,459
Operating expenses:
Rents and purchased transportation	4,434,540	3,650,806	3,255,692
Salaries, wages and employee benefits	1,926,213	1,608,378	1,469,187
Fuel and fuel taxes	459,011	347,573	283,437
Depreciation and amortization	435,893	383,518	361,510
Operating supplies and expenses	303,529	257,239	233,223
General and administrative expenses, net of asset dispositions	163,270	125,878	87,053
Insurance and claims	129,406	123,579	78,410
Operating taxes and licenses	51,080	44,825	45,954
Communication and utilities	30,911	23,983	19,973
Total operating expenses	7,933,853	6,565,779	5,834,439
Operating income	681,021	623,789	721,020
Interest income	224	235	71
Interest expense	40,427	28,785	25,294
Earnings before income taxes	640,818	595,239	695,797
Income taxes	151,233	(91,024)	263,707
Net earnings	$ 489,585	$ 686,263	$ 432,090
Weighted average basic shares outstanding (in shares)	109,375	109,987	112,474
Basic earnings per share (in dollars per share)	$ 4.48	$ 6.24	$ 3.84
Weighted average diluted shares outstanding (in shares)	110,428	111,049	113,361
Diluted earnings per share (in dollars per share)	$ 4.43	$ 6.18	$ 3.81
Dividends declared per common share (in dollars per share)	$ 0.96	$ 0.92	$ 0.88
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 7,557,648	$ 6,435,858	$ 6,007,347
Fuel Surcharge [Member]
Operating revenues	$ 1,057,226	$ 753,710	$ 548,112