SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RECENTLY ADOPTED ACCOUNTING STANDARDS (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Deferred taxes (current)		$ 0
Total current assets	$ 607,264	488,215
Other noncurrent assets	68,708	61,915
Total noncurrent assets	712,166	667,881
Total assets	3,727,082	3,284,357	$ 3,125,388
Deferred income taxes	473,847	444,935
Total noncurrent liabilities	925,333	897,706
Total capitalization and liabilities	3,727,082	3,284,357
Excess tax benefits, stock compensation	1,800
Tax benefit from stock based compensation	1,755	881	414
Other noncurrent liabilities	21,336	27,841	20,724
Net cash flows provided from operating activities		390,947	358,149
Tax benefit from stock options exercised		0	0
Tax withholding payments related to net settled stock compensation	(3,547)	(2,146)	(949)
Cash flows (used in) financing activities	$ 253,178	(66,441)	(76,372)
As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Deferred taxes (current)		56,296
Total current assets		544,511
Other noncurrent assets		60,300
Total noncurrent assets		666,266
Total assets		3,339,038
Deferred income taxes		499,616
Total noncurrent liabilities		952,387
Total capitalization and liabilities		3,339,038
Tax benefit from stock based compensation		0	0
Other noncurrent liabilities		25,695	19,775
Net cash flows provided from operating activities		387,920	356,786
Tax benefit from stock options exercised		881	414
Tax withholding payments related to net settled stock compensation		0	0
Cash flows (used in) financing activities		(63,414)	(75,009)
Effect of Change
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Deferred taxes (current)		(56,296)
Total current assets		(56,296)
Other noncurrent assets		1,615
Total noncurrent assets		1,615
Total assets		(54,681)
Deferred income taxes		(54,681)
Total noncurrent liabilities		(54,681)
Total capitalization and liabilities		(54,681)
Tax benefit from stock based compensation		881	414
Other noncurrent liabilities		2,146	949
Net cash flows provided from operating activities		3,027	1,363
Tax benefit from stock options exercised		(881)	(414)
Tax withholding payments related to net settled stock compensation		(2,146)	(949)
Cash flows (used in) financing activities		$ (3,027)	$ (1,363)