Regulatory Assets and Liabilities and Regulatory Matters - Regulatory Assets and Liabilities Associated with Gas Utility and Electric Utility (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Regulatory Assets
Regulatory assets	$ 301,002	$ 368,929
Regulatory Liabilities [Line Items]
Regulatory liabilities	390,175	49,230
Increase (decrease) in accounts payable, trade	3,987	2,107	$ (3,053)
Postretirement benefits overcollections
Regulatory Liabilities [Line Items]
Regulatory liabilities	17,781	17,493
Deferred fuel and power refunds
Regulatory Liabilities [Line Items]
Regulatory liabilities	36,723	10,621
State income tax benefits — distribution system repairs
Regulatory Liabilities [Line Items]
Regulatory liabilities	22,611	18,430
PUC temporary rates order
Regulatory Liabilities [Line Items]
Regulatory liabilities	24,430	0
Excess Federal Deferred Income Taxes
Regulatory Liabilities [Line Items]
Regulatory liabilities	285,221	0
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	3,409	2,686
Income taxes recoverable
Regulatory Assets
Regulatory assets	110,129	121,421
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets	87,106	141,310
Environmental costs
Regulatory Assets
Regulatory assets	58,836	61,566
Deferred fuel and power costs
Regulatory Assets
Regulatory assets	0	7,685
Removal costs, net
Regulatory Assets
Regulatory assets	32,025	30,996
Other
Regulatory Assets
Regulatory assets	$ 12,906	$ 5,951