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, 2019	Sep. 30, 2018	Jun. 30, 2018
Regulatory Assets
Regulatory assets	$ 300,677	$ 301,002	$ 360,061
Regulatory Liabilities
Regulatory liabilities	376,170	390,175	412,451
Income taxes recoverable
Regulatory Assets
Regulatory assets	123,901	110,129	130,024
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets	81,791	87,106	132,239
Environmental costs
Regulatory Assets
Regulatory assets	56,561	58,836	59,808
Removal costs, net
Regulatory Assets
Regulatory assets	29,339	32,025	30,987
Other
Regulatory Assets
Regulatory assets	9,085	12,906	7,003
Postretirement benefits
Regulatory Liabilities
Regulatory liabilities	16,481	17,781	16,895
Deferred fuel and power refunds
Regulatory Liabilities
Regulatory liabilities	12,416	36,723	44,500
State tax benefits — distribution system repairs
Regulatory Liabilities
Regulatory liabilities	25,176	22,611	20,677
PAPUC temporary rates order
Regulatory Liabilities
Regulatory liabilities	25,414	24,430	24,098
Excess federal deferred income taxes
Regulatory Liabilities
Regulatory liabilities	282,735	285,221	301,151
Other
Regulatory Liabilities
Regulatory liabilities	$ 13,948	$ 3,409	$ 5,130