Income Taxes - Deferred Tax Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Excess book basis over tax basis of property, plant and equipment	$ 491,038	$ 431,480
Goodwill	45,070	40,552
Derivative financial instruments	948	0
Regulatory assets	149,660	117,420
Other	2,910	2,573
Gross deferred tax liabilities	689,626	592,025
Pension plan liabilities	(74,129)	(54,444)
Allowance for doubtful accounts	(1,637)	(2,809)
Deferred investment tax credits	(1,356)	(1,493)
Employee-related expenses	(5,247)	(5,637)
Regulatory liabilities	(16,798)	(23,958)
Environmental liabilities	(22,757)	(6,014)
Derivative financial instruments	0	(3,501)
Other	(17,473)	(6,367)
Gross deferred tax assets	(139,397)	(104,223)
Net deferred tax liabilities	$ 550,229	$ 487,802