SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NEW ACCOUNTING STANDARD (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Sep. 30, 2015
Assets
Deferred taxes (current)		$ 0
Total current assets	$ 588,939	488,215
Other noncurrent assets	65,057	61,915
Total noncurrent assets	692,200	667,881
Total assets	3,477,238	3,284,357
Capitalization and Liabilities
Deferred income taxes	491,522	444,935
Total noncurrent liabilities	910,106	897,706
Total capitalization and liabilities	$ 3,477,238	3,284,357
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
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)