Income Taxes - Deferred Tax Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Excess book basis over tax basis of property, plant and equipment	$ 441,154	$ 564,327
Goodwill	37,414	49,588
Derivative financial instruments	579	0
Regulatory assets	90,022	136,093
Other	3,375	3,140
Gross deferred tax liabilities	572,544	753,148
Pension plan liabilities	(19,831)	(57,011)
Allowance for doubtful accounts	(2,820)	(1,681)
Deferred investment tax credits	(760)	(1,224)
Employee-related expenses	(4,581)	(6,793)
Regulatory liabilities	(118,506)	(12,780)
Environmental liabilities	(14,551)	(22,224)
Derivative financial instruments	0	(354)
Other	(10,571)	(15,616)
Gross deferred tax assets	(171,620)	(117,683)
Net deferred tax liabilities	$ 400,924	$ 635,465