Income Taxes - Deferred Tax Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Excess book basis over tax basis of property, plant and equipment	$ 564,327	$ 491,038
Goodwill	49,588	45,070
Derivative financial instruments	0	948
Regulatory assets	136,093	149,660
Other	3,140	2,910
Gross deferred tax liabilities	753,148	689,626
Pension plan liabilities	(57,011)	(74,129)
Allowance for doubtful accounts	(1,681)	(1,637)
Deferred investment tax credits	(1,224)	(1,356)
Employee-related expenses	(6,793)	(5,247)
Regulatory liabilities	(12,780)	(16,798)
Environmental liabilities	(22,224)	(22,757)
Derivative financial instruments	(354)	0
Other	(15,616)	(17,473)
Gross deferred tax assets	(117,683)	(139,397)
Net deferred tax liabilities	$ 635,465	$ 550,229