REVENUE RECOGNITION Revenue recognition adoption impact (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	$ 321,580	$ 340,593	$ 660,046	$ 662,711
Operating revenue from affiliates	13,131	17,399	25,652	35,010
Reimbursable revenue from non-affiliates	15,946	15,684	32,853	28,064
Revenue	350,657	$ 373,676	718,551	$ 725,785
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	350,657		718,551
Previously Reported [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	350,657		718,551
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	0		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	317,369		642,725
Operating revenue from affiliates	5,133		10,927
Reimbursable revenue from non-affiliates	15,946		32,853
Revenue	338,448		686,505
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	321,580		660,046
Operating revenue from affiliates	13,131		25,652
Reimbursable revenue from non-affiliates	15,946		32,853
Revenue	350,657		718,551
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	(4,211)		(17,321)
Operating revenue from affiliates	(7,998)		(14,725)
Reimbursable revenue from non-affiliates	0		0
Revenue	(12,209)		(32,046)
Revenue Not from Contract with Customer [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	12,200		32,000
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	4,211		17,321
Operating revenue from affiliates	7,998		14,725
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		0
Operating revenue from affiliates	0		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	4,211		17,321
Operating revenue from affiliates	$ 7,998		$ 14,725