REVENUE - Contract Balances (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2019	Dec. 31, 2018
Contract with Customer, Asset and Liability [Abstract]
Trade accounts receivable	[1]	$ 265,104	$ 268,352
Contract assets	[2]	7,367	5,745
Contract liabilities	[3]	1,738	$ 1,784
Change in Contract with Customer, Asset [Abstract]
Contract asset, increase from beginning of period		$ 1,600

[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 sheets.

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