CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Transportation	$ 13,502,906	$ 11,704,745	$ 11,989,780
Sourcing	1,366,474	1,439,668	1,486,304
Total revenues	14,869,380	13,144,413	13,476,084
Costs and expenses:
Purchased transportation and related services	11,257,290	9,549,934	9,842,271
Purchased products sourced for resale	1,244,040	1,316,951	1,365,333
Personnel expenses	1,179,527	1,064,936	1,051,410
Other selling, general, and administrative expenses	413,404	375,061	358,760
Total costs and expenses	14,094,261	12,306,882	12,617,774
Income from operations	775,119	837,531	858,310
Interest and other expense	(46,656)	(25,581)	(35,529)
Income before provision for income taxes	728,463	811,950	822,781
Provision for income taxes	223,570	298,566	313,082
Net income	504,893	513,384	509,699
Other comprehensive income/(loss)	42,982	(23,496)	(9,336)
Comprehensive income	$ 547,875	$ 489,888	$ 500,363
Basic net income per share (in dollars per share)	$ 3.59	$ 3.60	$ 3.52
Diluted net income per share (in dollars per share)	$ 3.57	$ 3.59	$ 3.51
Basic weighted average shares outstanding (in shares)	140,610	142,706	144,967
Dilutive effect of outstanding stock awards (in shares)	772	285	382
Diluted weighted average shares outstanding (in shares)	141,382	142,991	145,349