Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Operating revenues, excluding fuel surcharge revenues	$ 1,551,051	$ 1,483,354	$ 3,012,820	$ 2,910,008
Fuel surcharge revenues	175,864	131,672	343,253	233,730
Total operating revenues	1,726,915	1,615,026	3,356,073	3,143,738
Operating expenses:
Rents and purchased transportation	871,122	794,907	1,677,562	1,535,310
Salaries, wages and employee benefits	389,873	371,969	770,184	734,480
Depreciation and amortization	93,050	90,364	185,238	178,716
Fuel and fuel taxes	79,072	71,489	159,719	130,903
Operating supplies and expenses	64,486	56,495	122,507	111,031
Insurance and claims	27,461	19,094	50,466	36,522
General and administrative expenses, net of asset dispositions	21,728	18,711	45,209	40,545
Operating taxes and licenses	10,905	11,365	21,585	22,491
Communication and utilities	5,603	4,840	10,599	10,058
Total operating expenses	1,563,300	1,439,234	3,043,069	2,800,056
Operating income	163,615	175,792	313,004	343,682
Net interest expense	7,393	6,420	14,211	12,862
Earnings before income taxes	156,222	169,372	298,793	330,820
Income taxes	58,353	64,361	98,222	125,711
Net earnings	$ 97,869	$ 105,011	$ 200,571	$ 205,109
Weighted average basic shares outstanding (in shares)	109,630	112,669	110,250	112,871
Basic earnings per share (in dollars per share)	$ 0.89	$ 0.93	$ 1.82	$ 1.82
Weighted average diluted shares outstanding (in shares)	110,822	113,761	111,420	113,882
Diluted earnings per share (in dollars per share)	$ 0.88	$ 0.92	$ 1.80	$ 1.80
Dividends declared per common share (in dollars per share)	$ 0.23	$ 0.22	$ 0.46	$ 0.44