SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 378,615	$ 333,890	$ 247,666	$ 168,229
Stock award, vesting period	5 years
Minimum
Significant Accounting Policies [Line Items]
Intangible assets, estimated lives	5 years
Maximum
Significant Accounting Policies [Line Items]
Intangible assets, estimated lives	8 years
Full Value Awards
Significant Accounting Policies [Line Items]
Stock award, vesting period	5 years
Full Value Awards | Minimum
Significant Accounting Policies [Line Items]
Shares and restricted units grants, discount for post-vesting holding restrictions (percent)	15.00%
Full Value Awards | Maximum
Significant Accounting Policies [Line Items]
Shares and restricted units grants, discount for post-vesting holding restrictions (percent)	21.00%
Software
Significant Accounting Policies [Line Items]
Intangible assets, estimated lives	3 years
Held outside the United States
Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 320,000	$ 275,300
Transportation Services
Significant Accounting Policies [Line Items]
General payment terms upon completion of performance obligation	30 days
Value-Added Logistics Services
Significant Accounting Policies [Line Items]
General payment terms upon completion of performance obligation	30 days
Sourcing Services
Significant Accounting Policies [Line Items]
General payment terms upon completion of performance obligation	30 days