Income Taxes - Deferred Tax Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Sep. 30, 2014
Income Tax Disclosure [Abstract]
Excess book basis over tax basis of property, plant and equipment	$ 431,480	$ 392,839
Goodwill	40,552	36,034
Regulatory assets	117,420	109,953
Other	2,573	1,349
Gross deferred tax liabilities	592,025	540,175
Pension plan liabilities	(54,444)	(40,461)
Allowance for doubtful accounts	(2,809)	(2,903)
Deferred investment tax credits	(1,493)	(1,632)
Employee-related expenses	(5,637)	(5,630)
Regulatory liabilities	(23,958)	(14,836)
Environmental liabilities	(6,014)	(4,389)
Derivative financial instruments	(3,501)	(6,224)
Other	(6,367)	(4,131)
Gross deferred tax assets	(104,223)	(80,206)
Net deferred tax liabilities	$ 487,802	$ 459,969