REVENUE - Contract Balances (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2018		Jan. 01, 2018		Dec. 31, 2017
Contract with Customer, Asset and Liability [Abstract]
Trade accounts receivable, net		$ 301,912	[1]	$ 301,963	[1]	$ 301,963
Contract assets	[2]	7,765		9,823
Contract liabilities		1,457	[3]	$ 5,415	[3]	$ 6,102
Change in Contract with Customer, Asset [Abstract]
Contract asset, decrease from beginning of period		$ 2,100

[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.