REVENUE RECOGNITION Revenue recognition adoption impact (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenue from non-affiliates	$ 303,206	$ 328,944	$ 963,252	$ 991,655
Operating revenue from affiliates	13,885	16,584	39,537	51,594
Reimbursable revenue from non-affiliates	14,238	15,207	47,091	43,271
Revenue	331,329	$ 360,735	1,049,880	$ 1,086,520
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	331,329		1,049,880
Previously Reported [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	331,329		1,049,880
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	301,439		944,164
Operating revenue from affiliates	5,895		16,822
Reimbursable revenue from non-affiliates	14,238		47,091
Revenue	321,572		1,008,077
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	303,206		963,252
Operating revenue from affiliates	13,885		39,537
Reimbursable revenue from non-affiliates	14,238		47,091
Revenue	331,329		1,049,880
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	(1,767)		(19,088)
Operating revenue from affiliates	(7,990)		(22,715)
Reimbursable revenue from non-affiliates	0		0
Revenue	(9,757)		(41,803)
Revenue Not from Contract with Customer [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	9,800		41,800
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	1,767		19,088
Operating revenue from affiliates	7,990		22,715
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	1,767		19,088
Operating revenue from affiliates	$ 7,990		$ 22,715