Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating revenues	$ 2,139,027	$ 1,726,915	$ 4,087,272	$ 3,356,073
Operating expenses:
Rents and purchased transportation	1,073,164	871,122	2,038,057	1,677,562
Salaries, wages and employee benefits	465,326	389,873	915,592	770,184
Fuel and fuel taxes	115,541	79,072	223,422	159,719
Depreciation and amortization	107,423	93,050	213,006	185,238
Operating supplies and expenses	76,446	64,486	147,126	122,507
General and administrative expenses, net of asset dispositions	37,306	21,728	69,630	45,209
Insurance and claims	28,371	27,461	56,870	50,466
Operating taxes and licenses	12,234	10,905	23,822	21,585
Communication and utilities	8,404	5,603	16,153	10,599
Total operating expenses	1,924,215	1,563,300	3,703,678	3,043,069
Operating income	214,812	163,615	383,594	313,004
Net interest expense	9,855	7,393	19,008	14,211
Earnings before income taxes	204,957	156,222	364,586	298,793
Income taxes	53,305	58,353	94,792	98,222
Net earnings	$ 151,652	$ 97,869	$ 269,794	$ 200,571
Weighted average basic shares outstanding (in shares)	109,449	109,630	109,601	110,250
Basic earnings per share (in dollars per share)	$ 1.39	$ 0.89	$ 2.46	$ 1.82
Weighted average diluted shares outstanding (in shares)	110,682	110,822	110,771	111,420
Diluted earnings per share (in dollars per share)	$ 1.37	$ 0.88	$ 2.44	$ 1.80
Dividends declared per common share (in dollars per share)	$ 0.24	$ 0.23	$ 0.48	$ 0.46
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 1,874,388	$ 1,551,051	$ 3,587,321	$ 3,012,820
Fuel Surcharge [Member]
Operating revenues	$ 264,639	$ 175,864	$ 499,951	$ 343,253