STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012
Statement of Other Comprehensive Income [Abstract]
Net Income	$ 29,775	$ 25,925	$ 55,517	$ 47,622
Net gains (losses) on cash flow hedging derivative instruments:
Net hedging gain arising during the period	147	83	203	133
Reclassification adjustment for (gains) losses included in net income	(38)	11	(85)	(3)
Net unrealized gains on cash flow hedging derivative instruments	109	94	118	130
Defined benefit pension and other postretirement benefit plans:
Net actuarial loss arising during the period	0	(2,366)	0	(2,366)
Amortization of actuarial loss included in net periodic pension and postretirement benefit cost	90	3,482	181	3,573
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Adjustment, before Tax	90	1,116	181	1,207
Other Comprehensive Income, Before Tax	199	1,210	299	1,337
Income Tax Expense Related to Items of Other Comprehensive Income	76	467	124	516
Other Comprehensive Income, Net of Tax	123	743	175	821
Comprehensive Income	$ 29,898	$ 26,668	$ 55,692	$ 48,443