Condensed Consolidated Statements of Earnings (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating revenues	$ 2,209,760	$ 1,843,334	$ 6,297,032	$ 5,199,408
Operating expenses:
Rents and purchased transportation	1,125,386	947,145	3,163,443	2,624,707
Salaries, wages and employee benefits	495,350	408,340	1,410,942	1,178,524
Fuel and fuel taxes	117,976	87,006	341,398	246,725
Depreciation and amortization	108,801	95,959	321,807	281,198
Operating supplies and expenses	79,225	67,578	226,352	190,085
General and administrative expenses, net of asset dispositions	42,449	29,389	112,078	74,597
Insurance and claims	45,594	26,463	102,464	76,930
Operating taxes and licenses	13,203	10,744	37,025	32,329
Communication and utilities	7,088	5,738	23,241	16,337
Total operating expenses	2,035,072	1,678,362	5,738,750	4,721,432
Operating income	174,688	164,972	558,282	477,976
Net interest expense	9,961	8,310	28,969	22,521
Earnings before income taxes	164,727	156,662	529,313	455,455
Income taxes	33,617	56,277	128,409	154,499
Net earnings	$ 131,110	$ 100,385	$ 400,904	$ 300,956
Weighted average basic shares outstanding (in shares)	109,254	109,703	109,484	110,066
Basic earnings per share (in dollars per share)	$ 1.20	$ 0.92	$ 3.66	$ 2.73
Weighted average diluted shares outstanding (in shares)	110,235	110,628	110,591	111,154
Diluted earnings per share (in dollars per share)	$ 1.19	$ 0.91	$ 3.63	$ 2.71
Dividends declared per common share (in dollars per share)	$ 0.24	$ 0.23	$ 0.72	$ 0.69
Service Excluding Fuel Surcharge [Member]
Operating revenues	$ 1,936,653	$ 1,657,380	$ 5,523,974	$ 4,670,200
Fuel Surcharge [Member]
Operating revenues	$ 273,107	$ 185,954	$ 773,058	$ 529,208