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ACCOUNTING POLICIES (Not Yet Adopted Pronouncements) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
ASC Topic 606      
Retained earnings   $ 438,379 $ 386,917
ASC Topic 606, Revenue Recognition | Forecast | Difference between Revenue Guidance in Effect before and after Topic 606 | Maximum      
ASC Topic 606      
Retained earnings $ 500