SUPPLEMENTARY DATA (UNAUDITED) - Summary of Unaudited Results of Operations by Quarter (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Total revenues	$ 4,137,908	$ 4,291,900	$ 4,276,037	$ 3,925,327	$ 3,959,786	$ 3,784,451	$ 3,710,018	$ 3,415,125	$ 16,631,172	$ 14,869,380	$ 13,144,413
Costs and expenses:
Personnel expenses	339,316	335,299	340,630	328,297	311,599	293,204	284,220	290,504	1,343,542	1,179,527	1,064,936
Other selling, general, and administrative expenses	118,950	112,772	111,845	106,043	109,374	106,177	107,749	90,104	449,610	413,404	375,061
Total costs and expenses	3,882,391	4,045,927	4,057,029	3,733,742	3,748,910	3,589,986	3,528,198	3,227,167	15,719,089	14,094,261	12,306,882
Income from operations	255,517	245,973	219,008	191,585	210,876	194,465	181,820	187,958	912,083	775,119	837,531
Net income	$ 187,150	$ 175,895	$ 159,163	$ 142,297	$ 152,556	$ 119,186	$ 111,071	$ 122,080	$ 664,505	$ 504,893	$ 513,384
Basic net income per share (in dollars per share)	$ 1.36	$ 1.27	$ 1.14	$ 1.02	$ 1.09	$ 0.85	$ 0.79	$ 0.86	$ 4.78	$ 3.59	$ 3.60
Diluted net income per share (in dollars per share)	$ 1.34	$ 1.25	$ 1.13	$ 1.01	$ 1.08	$ 0.85	$ 0.78	$ 0.86	$ 4.73	$ 3.57	$ 3.59
Basic weighted average shares outstanding (in shares)	137,797	138,797	139,464	140,032	139,572	140,422	141,061	141,484	139,010	140,610	142,706
Dilutive effect of outstanding stock awards (in shares)	1,385	1,363	1,147	1,238	1,152	600	526	374	1,395	772	285
Diluted weighted average shares outstanding (in shares)	139,182	140,160	140,611	141,270	140,724	141,022	141,587	141,858	140,405	141,382	142,991
Income tax expense (benefit)									$ 215,768	$ 223,570	$ 298,566
Decrease in provision for income taxes related to section 199 deductions					$ 19,700
Decrease in provision for income taxes due to impact of the Tax Act					12,100
Transportation
Revenues:
Total revenues	$ 3,896,750	$ 4,028,392	$ 3,953,139	$ 3,637,640	3,647,167	$ 3,433,701	$ 3,319,995	$ 3,102,043	15,515,921	13,502,906	11,704,745
Costs and expenses:
Purchased services and products	3,207,859	3,359,520	3,313,196	3,041,602	3,042,434	2,869,616	2,781,355	2,563,885	12,922,177	11,257,290	9,549,934
Sourcing
Revenues:
Total revenues	241,158	263,508	322,898	287,687	312,619	350,750	390,023	313,082	1,115,251	1,366,474	1,439,668
Costs and expenses:
Purchased services and products	216,266	238,336	291,358	257,800	$ 285,503	$ 320,989	$ 354,874	282,674	$ 1,003,760	$ 1,244,040	$ 1,316,951
ASU No. 2014-09
Costs and expenses:
Net income	$ (4,200)	$ 500	$ 6,600	$ 2,100
ASU No. 2016-09
Costs and expenses:
Income tax expense (benefit)								$ (13,700)