SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NEW ACCOUNTING STANDARD (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016
Income Statement [Abstract]
Income tax provision	$ 2,018	$ 6,722
Net income	$ 34,929	$ 50,282
EARNINGS PER COMMON SHARE
Basic (usd per share)	$ 0.41	$ 0.59
Diluted (usd per share)	$ 0.40	$ 0.58
Cash flows (used in) operating activities
Net income, as reported	$ 34,929	$ 50,282
Tax benefit from stock-based compensation	1,188	1,635
Other noncurrent liabilities	15	4,173
Net cash flows (used in) operating activities	(46,064)	(36,174)
Cash flows from financing activities
Tax withholding payments related to net settled stock compensation	(4,167)	(3,042)
Cash flows from financing activities	141,934	128,314
Assets
Other noncurrent assets	60,937		$ 60,196
Total noncurrent assets	682,694		703,654
Total assets	3,949,506		3,718,570
Capitalization and Liabilities
Long-term debt	1,026,682		1,055,038
Total capitalization	2,212,043		2,221,629
Total capitalization and liabilities	3,949,506		3,718,570
As Previously Reported
Income Statement [Abstract]
Income tax provision		8,357
Net income		$ 48,647
EARNINGS PER COMMON SHARE
Basic (usd per share)		$ 0.57
Diluted (usd per share)		$ 0.56
Cash flows (used in) operating activities
Net income, as reported		$ 48,647
Tax benefit from stock-based compensation		0
Other noncurrent liabilities		4,401
Net cash flows (used in) operating activities		(39,216)
Cash flows from financing activities
Tax withholding payments related to net settled stock compensation		0
Cash flows from financing activities		131,356
Assets
Other noncurrent assets			68,708
Total noncurrent assets			712,166
Total assets			3,727,082
Capitalization and Liabilities
Long-term debt			1,063,550
Total capitalization			2,230,141
Total capitalization and liabilities			3,727,082
Effect of Change
Income Statement [Abstract]
Income tax provision		(1,635)
Net income		$ 1,635
EARNINGS PER COMMON SHARE
Basic (usd per share)		$ 0.02
Diluted (usd per share)		$ 0.02
Cash flows (used in) operating activities
Net income, as reported		$ 1,635
Tax benefit from stock-based compensation		1,635
Other noncurrent liabilities		(228)
Net cash flows (used in) operating activities		3,042
Cash flows from financing activities
Tax withholding payments related to net settled stock compensation		(3,042)
Cash flows from financing activities		$ (3,042)
Assets
Other noncurrent assets			(8,512)
Total noncurrent assets			(8,512)
Total assets			(8,512)
Capitalization and Liabilities
Long-term debt			(8,512)
Total capitalization			(8,512)
Total capitalization and liabilities			$ (8,512)
Other Noncurrent Assets | Accounting Standards Update 2015-03
New Accounting Pronouncement, Early Adoption [Line Items]
Debt issuance costs	(8,400)
Long-term Debt | Accounting Standards Update 2015-03
New Accounting Pronouncement, Early Adoption [Line Items]
Debt issuance costs	7,400
Short-term Debt | Accounting Standards Update 2015-03
New Accounting Pronouncement, Early Adoption [Line Items]
Debt issuance costs	$ 949