INCOME TAXES, INCOME TAX RECONCILIATION (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Income Tax Disclosure [Abstract]
Federal statutory rate	35.00%	35.00%	35.00%
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory income tax expense	$ 84,239	$ 67,834	$ 52,661
Change resulting from
State income taxes	8,233	7,785	5,168
Depreciation and cost of removal	(5,149)	(4,437)	(5,769)
Investment/production tax credits	(30,096)	(23,083)	(18,749)
Basis adjustment of solar assets due to ITC	4,861	3,959	3,225
Other	(2,364)	(218)	(961)
Income tax provision	$ 59,724	$ 51,840	$ 35,575
Effective income tax rate	24.80%	26.80%	23.60%