Income Taxes (Deferred tax liabilities (assets)) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Deferred tax liabilities (assets)
Excess book basis over tax basis of property, plant and equipment	$ 362,259	$ 330,937
Goodwill	31,516	26,998
Regulatory assets	101,622	140,416
Other	949	601
Gross deferred tax liabilities	496,346	498,952
Pension plan liabilities	(35,996)	(72,652)
Allowance for doubtful accounts	(2,290)	(1,489)
Deferred investment tax credits	(1,772)	(1,914)
Employee-related expenses	(5,850)	(4,859)
Regulatory liabilities	(15,472)	(11,761)
Environmental liabilities	(7,002)	(7,195)
Derivative financial instruments	(3,397)	(16,264)
Other	(1,922)	(12,202)
Gross deferred tax assets	(73,701)	(128,336)
Net deferred tax liabilities	$ 422,645	$ 370,616