REVENUE - Contract Balances (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Dec. 31, 2018
Contract with Customer, Asset and Liability [Abstract]
Trade accounts receivable	[1]	$ 244,306	$ 268,352
Contract assets	[2]	6,748	5,745
Contract liabilities	[3]	2,014	$ 1,784
Change in Contract with Customer, Asset [Abstract]
Contract asset, increase from beginning of period		1,000
Change in Contract with Customer, Liability [Abstract]
Contract liability, increase from beginning of period		$ 200

[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.