Other Income, Net (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010
Component of Other Income, Nonoperating [Line Items]
Total other income, net	$ 4,989	$ 10,764	$ 6,269
Non-tariff service income [Member]
Component of Other Income, Nonoperating [Line Items]
Total other income, net	2,653	6,422	2,437
Interest income [Member]
Component of Other Income, Nonoperating [Line Items]
Total other income, net	572	467	867
Postretirement benefit plan curtailment gain [Member]
Component of Other Income, Nonoperating [Line Items]
Total other income, net	0	3,245	0
Other [Member]
Component of Other Income, Nonoperating [Line Items]
Total other income, net	$ 1,764	$ 630	$ 2,965