REVENUE RECOGNITION Revenue recognition adoption impact (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	$ 291,348	$ 321,580	$ 595,478	$ 660,046
Operating revenue from affiliates	13,336	11,817	25,782	23,112
Reimbursable revenue from non-affiliates	13,536	15,946	30,136	32,853
Revenue	318,220	349,343	651,396	716,011
Revenue from Contract with Customer Benchmark [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	290,939	317,369	594,672	642,725
Operating revenue from affiliates	5,365	3,819	9,840	8,387
Reimbursable revenue from non-affiliates	13,536	15,946	30,136	32,853
Revenue	309,840	337,134	634,648	683,965
Revenue Not from Contract with Customer [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	409	4,211	806	17,321
Operating revenue from affiliates	$ 7,971	$ 7,998	$ 15,942	$ 14,725