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REVENUE RECOGNITION Revenue recognition adoption impact (Details) - USD ($)
$ in Thousands
3 Months Ended
Jun. 30, 2019
Jun. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Operating revenue from non-affiliates $ 304,130 $ 338,466
Operating revenue from affiliates 12,446 11,295
Reimbursable revenue from non-affiliates 16,600 16,907
Revenue 333,176 366,668
Revenue from Contract with Customer Benchmark [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Operating revenue from non-affiliates 303,733 325,356
Operating revenue from affiliates 4,475 4,568
Reimbursable revenue from non-affiliates 16,600 16,907
Revenue 324,808 346,831
Revenue Not from Contract with Customer [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Operating revenue from non-affiliates 397 13,110
Operating revenue from affiliates $ 7,971 $ 6,727