Income Taxes (Deferred tax liabilities (assets)) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2011
Deferred tax liabilities (assets)
Excess book basis over tax basis of property, plant and equipment	$ 330,937	$ 293,457
Goodwill	26,998	22,446
Regulatory assets	140,416	124,669
Other	601	487
Gross deferred tax liabilities	498,952	441,059
Pension plan liabilities	(72,652)	(62,818)
Allowance for doubtful accounts	(1,489)	(2,642)
Deferred investment tax credits	(1,914)	(2,057)
Employee-related expenses	(4,859)	(8,032)
Regulatory liabilities	(11,761)	(12,378)
Environmental liabilities	(7,195)	(7,749)
Derivative financial instruments	(16,264)	(11,931)
Other	(12,202)	(14,512)
Gross deferred tax assets	(128,336)	(122,119)
Net deferred tax liabilities	$ 370,616	$ 318,940