Regulatory Assets and Liabilities and Regulatory Matters - Schedule of Regulatory Assets and Liabilities Associated With Gas Utility and Electric Utility (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
Regulatory Assets
Regulatory assets	$ 360,061	$ 368,929	$ 398,747
Regulatory Liabilities
Regulatory liabilities	412,451	49,230	48,670
Income taxes recoverable
Regulatory Assets
Regulatory assets	130,024	121,421	122,733
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets	132,239	141,310	171,833
Environmental costs
Regulatory Assets
Regulatory assets	59,808	61,566	61,616
Deferred fuel and power costs
Regulatory Assets
Regulatory assets	190	7,685	7,024
Removal costs, net
Regulatory Assets
Regulatory assets	30,987	30,996	29,405
Other
Regulatory Assets
Regulatory assets	6,813	5,951	6,136
Postretirement benefits
Regulatory Liabilities
Regulatory liabilities	16,895	17,493	16,715
Deferred fuel and power refunds
Regulatory Liabilities
Regulatory liabilities	44,500	10,621	12,587
State tax benefits — distribution system repairs
Regulatory Liabilities
Regulatory liabilities	20,677	18,430	16,662
PUC Temporary Rates Order (a)
Regulatory Liabilities
Regulatory liabilities	24,098	0	0
Excess federal deferred income taxes (b)
Regulatory Liabilities
Regulatory liabilities	301,151	0	0
Other
Regulatory Liabilities
Regulatory liabilities	$ 5,130	$ 2,686	$ 2,706