EMPLOYEE BENEFIT PLANS, Defined Benefit Plans (Details) (USD $)			3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended
	Mar. 31, 2012	Sep. 30, 2011	Mar. 31, 2012 Pension [Member]	Mar. 31, 2011 Pension [Member]	Mar. 31, 2012 Pension [Member]	Mar. 31, 2011 Pension [Member]	Mar. 31, 2012 Pension Plans Excluding Pension Equalization Plan, Defined Benefit [Member]	Mar. 31, 2012 OPEB [Member]	Mar. 31, 2011 OPEB [Member]	Mar. 31, 2012 OPEB [Member]	Mar. 31, 2011 OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Contributions by Employer							$ 20,000,000
Change in Benefit Obligation [Roll Forward]
Service cost			1,344,000	1,194,000	2,688,000	2,388,000		896,000	836,000	1,792,000	1,672,000
Interest cost			2,206,000	2,095,000	4,412,000	4,189,000		1,283,000	1,212,000	2,566,000	2,423,000
Change in plan assets [Roll Forward]
Defined Benefit Plan, Contributions by Employer							20,000,000
Amounts recognized on Consolidated Balance Sheets, Postemployment benefit liability
Non-current	(95,176,000)	(114,305,000)
Amounts Recognized in Regulatory Assets and Accumulated Other Comprehensive Income [Roll Forward]
Recognized actuarial loss			1,254,000	987,000	2,508,000	1,973,000		724,000	653,000	1,448,000	1,306,000
Prior service cost amortization			11,000	12,000	22,000	24,000		6,000	19,000	12,000	38,000
Recognized net initial obligation			0	0	0	0		89,000	89,000	178,000	178,000
Components of net periodic cost
Service cost			1,344,000	1,194,000	2,688,000	2,388,000		896,000	836,000	1,792,000	1,672,000
Interest cost			2,206,000	2,095,000	4,412,000	4,189,000		1,283,000	1,212,000	2,566,000	2,423,000
Expected return on plan assets			(3,171,000)	(2,873,000)	(6,342,000)	(5,745,000)		(686,000)	(618,000)	(1,372,000)	(1,236,000)
Recognized actuarial loss			1,254,000	987,000	2,508,000	1,973,000		724,000	653,000	1,448,000	1,306,000
Prior service cost amortization			11,000	12,000	22,000	24,000		6,000	19,000	12,000	38,000
Recognized net initial obligation			0	0	0	0		89,000	89,000	178,000	178,000
Net periodic cost			$ 1,644,000	$ 1,415,000	$ 3,288,000	$ 2,829,000		$ 2,312,000	$ 2,191,000	$ 4,624,000	$ 4,381,000