REVENUE - Contract Balances (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018		Jan. 01, 2018		Dec. 31, 2017
Contract with Customer, Asset and Liability [Abstract]
Trade accounts receivable, net		$ 288,980	[1]	$ 301,963	[1]	$ 301,963
Contract assets	[2]	14,792		9,823
Contract liabilities		3,799	[3]	$ 5,415	[3]	$ 6,102
Change in Contract with Customer, Asset [Abstract]
Contract asset, increase from beginning of period		$ 5,000

[1] Includes billed and unbilled amounts, net of allowance for doubtful accounts. See Note 3 for details.

[2] Included in the “Prepaid expenses and other current assets” line on the condensed consolidated balance sheet.

[3] Included in the “Other accrued liabilities” line of the condensed consolidated balance sheet.