REVENUE FROM CONTRACTS WITH CUSTOMERS - Cumulative Effect of Changes to Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Receivables, net of allowance for doubtful accounts	$ 2,202,460	$ 2,012,212	$ 2,113,930
Contract assets	182,247	147,764	0
Prepaid expenses and other	63,374	67,137	63,116
Liabilities
Accounts payable	1,059,669	943,812	1,000,305
Accrued expenses - transportation expense	140,231	94,811	0
Accrued expenses - compensation	99,279	107,280	105,316
Accrued expenses - other accrued liabilities	66,987	55,477	58,229
Deferred tax liabilities	42,779	48,653
Equity
Retained earnings	3,617,324	3,446,332	3,437,093
Calculated under Revenue Guidance in Effect before Topic 606
Assets
Receivables, net of allowance for doubtful accounts	2,268,854		2,113,930
Contract assets	0		0
Prepaid expenses and other	62,004		63,116
Liabilities
Accounts payable	1,103,551		1,000,305
Accrued expenses - transportation expense	0		0
Accrued expenses - compensation	96,766		105,316
Accrued expenses - other accrued liabilities	70,054		58,229
Deferred tax liabilities	37,235		45,355
Equity
Retained earnings	3,601,440		$ 3,437,093
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Assets
Receivables, net of allowance for doubtful accounts	(66,394)	(101,718)
Contract assets	182,247	147,764
Prepaid expenses and other	1,370	4,021
Liabilities
Accounts payable	(43,882)	(56,493)
Accrued expenses - transportation expense	140,231	94,811
Accrued expenses - compensation	2,513	1,964
Accrued expenses - other accrued liabilities	(3,067)	(2,752)
Deferred tax liabilities	5,544	3,298
Equity
Retained earnings	$ 15,884	$ 9,239