BUSINESS SEGMENT AND GEOGRAPHIC INFORMATION - Revenue from external customers (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Segment information
Operating revenues	$ 696,825	$ 648,872	$ 577,484	$ 564,087	$ 532,304	$ 498,564	$ 405,283	$ 368,590	$ 2,487,268	$ 1,804,741	$ 1,624,232
Property, plant and equipment, net	4,857,382				5,001,051				4,857,382	5,001,051	5,144,733
United States
Segment information
Operating revenues									2,247,400	1,591,769	1,386,786
Property, plant and equipment, net	4,591,913				4,686,235				4,591,913	4,686,235	4,804,328
Argentina
Segment information
Operating revenues									190,038	157,257	159,427
Property, plant and equipment, net	133,617				155,978				133,617	155,978	183,286
Colombia
Segment information
Operating revenues									38,793	37,554	20,488
Property, plant and equipment, net	74,042				81,798				74,042	81,798	91,815
Ecuador
Segment information
Operating revenues										6	4,948
Property, plant and equipment, net	10,781				22,298				10,781	22,298	438
Other Foreign
Segment information
Operating revenues									11,037	18,155	52,583
Property, plant and equipment, net	$ 47,029				$ 54,742				$ 47,029	$ 54,742	$ 64,866