STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Statement of Income and Comprehensive Income [Abstract]
Net Income	$ 25,925	$ 26,232	$ 47,622	$ 47,687
Net gains (losses) on cash flow hedging derivative instruments:
Net hedging gain arising during period	83	267	133	440
Reclassification adjustment for losses (gains) included in net income	11	(72)	(3)	(121)
Net unrealized gains on cash flow hedging derivative instruments	94	195	130	319
Defined benefit pension and other postretirement benefit plans:
Net actuarial loss arising during the period	(2,366)	0	(2,366)	0
Amortization of actuarial loss included in net periodic pension and postretirement benefit cost	3,482	106	3,573	213
Net defined benefit pension and other postretirement plans	1,116	106	1,207	213
Other Comprehensive Income, Before Tax	1,210	301	1,337	532
Income Tax Expense Related to Items of Other Comprehensive Income	467	116	516	205
Other Comprehensive Income, Net of Tax	743	185	821	327
Comprehensive Income	$ 26,668	$ 26,417	$ 48,443	$ 48,014