REVENUE RECOGNITION Revenue recognition adoption impact (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	$ 338,466	$ 322,118
Operating revenue from affiliates	12,521	17,611
Reimbursable revenue from non-affiliates	16,907	12,380
Revenue	367,894	$ 352,109
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	367,894
Previously Reported [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	367,894
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	0
Revenue from Contract with Customer [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	325,356
Operating revenue from affiliates	5,794
Reimbursable revenue from non-affiliates	16,907
Revenue	348,057
Revenue from Contract with Customer [Member] | Previously Reported [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	338,466
Operating revenue from affiliates	12,521
Reimbursable revenue from non-affiliates	16,907
Revenue	367,894
Revenue from Contract with Customer [Member] | Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	(13,110)
Operating revenue from affiliates	(6,727)
Reimbursable revenue from non-affiliates	0
Revenue	(19,837)
Revenue Not from Contract with Customer [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	19,800
Revenue Not from Contract with Customer [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	13,110
Operating revenue from affiliates	6,727
Revenue Not from Contract with Customer [Member] | Previously Reported [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	0
Operating revenue from affiliates	0
Revenue Not from Contract with Customer [Member] | Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	13,110
Operating revenue from affiliates	$ 6,727