ACQUISITIONS - Summary of Purchase Allocation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Jan. 01, 2015
Business Acquisition [Line Items]
Goodwill	$ 1,275,816	$ 1,232,796		$ 1,108,337
APC Logistics
Business Acquisition [Line Items]
Cash and cash equivalents			$ 10,181
Receivables			37,190
Other current assets			2,609
Property and equipment			1,696
Identifiable intangible assets			78,842
Goodwill			132,797
Other noncurrent assets			70
Long term deferred tax asset			814
Total assets			264,199
Accounts payable			(22,147)
Accrued expenses			(12,700)
Net assets acquired			$ 229,352
Freightquote
Business Acquisition [Line Items]
Cash and cash equivalents					$ 29,302
Receivables					56,228
Other current assets					2,395
Property and equipment					43,687
Identifiable intangible assets					37,800
Goodwill					287,220
Trademarks					8,600
Other noncurrent assets					3,421
Total assets					468,653
Accounts payable					(44,622)
Accrued expenses					(5,485)
Other liabilities					(19,939)
Net assets acquired					$ 398,607