SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NEW ACCOUNTING STANDARD (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Jun. 30, 2016	Jun. 30, 2016	Jun. 30, 2015	Sep. 30, 2015
Accounting Policies [Abstract]
Excess tax benefits associated with vesting of share-based awards	$ 41	$ 1,700
Assets
Deferred taxes (current)				$ 0
Total current assets	604,718	604,718		488,215
Other noncurrent assets	69,798	69,798		61,915
Total noncurrent assets	724,048	724,048		667,881
Total assets	3,632,790	3,632,790		3,284,357
Capitalization and Liabilities
Deferred income taxes	469,119	469,119		444,935
Total noncurrent liabilities	892,634	892,634		897,706
Total capitalization and liabilities	$ 3,632,790	3,632,790		3,284,357
Cash flows from operating activities
Tax benefit from stock based compensation		1,701	$ 838
Other noncurrent liabilities		9,656	36,350
Net cash flows provided from operating activities		96,599	382,734
Cash flows (used in) financing activities
Tax benefit from stock options exercised			0
Tax withholding payments related to net settled stock compensation		(3,359)	(2,136)
Cash flows (used in) financing activities		$ 248,613	(85,759)
As Previously Reported [Member]
Assets
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
Capitalization and Liabilities
Deferred income taxes				499,616
Total noncurrent liabilities				952,387
Total capitalization and liabilities				3,339,038
Cash flows from operating activities
Tax benefit from stock based compensation			0
Other noncurrent liabilities			34,214
Net cash flows provided from operating activities			379,760
Cash flows (used in) financing activities
Tax benefit from stock options exercised			838
Tax withholding payments related to net settled stock compensation			0
Cash flows (used in) financing activities			(82,785)
Effect of Change [Member]
Assets
Deferred taxes (current)				(56,296)
Total current assets				(56,296)
Other noncurrent assets				1,615
Total noncurrent assets				1,615
Total assets				(54,681)
Capitalization and Liabilities
Deferred income taxes				(54,681)
Total noncurrent liabilities				(54,681)
Total capitalization and liabilities				$ (54,681)
Cash flows from operating activities
Tax benefit from stock based compensation			838
Other noncurrent liabilities			2,136
Net cash flows provided from operating activities			2,974
Cash flows (used in) financing activities
Tax benefit from stock options exercised			(838)
Tax withholding payments related to net settled stock compensation			(2,136)
Cash flows (used in) financing activities			$ (2,974)