Regulatory Assets and Liabilities and Regulatory Matters - Schedule of Regulatory Assets and Liabilities Associated With Gas Utility and Electric Utility (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Regulatory Assets
Regulatory assets	$ 299,369	$ 301,002	$ 361,638
Regulatory Liabilities
Regulatory liabilities	379,524	390,175	380,721
Income taxes recoverable
Regulatory Assets
Regulatory assets	120,279	110,129	128,267
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets	83,563	87,106	135,263
Environmental costs
Regulatory Assets
Regulatory assets	58,758	58,836	59,788
Removal costs, net
Regulatory Assets
Regulatory assets	30,055	32,025	30,478
Other
Regulatory Assets
Regulatory assets	6,714	12,906	7,842
Postretirement benefits
Regulatory Liabilities
Regulatory liabilities	16,914	17,781	17,105
Deferred fuel and power refunds
Regulatory Liabilities
Regulatory liabilities	17,731	36,723	35,278
State tax benefits — distribution system repairs
Regulatory Liabilities
Regulatory liabilities	24,294	22,611	19,888
PAPUC temporary rates order
Regulatory Liabilities
Regulatory liabilities	25,098	24,430	0
Excess federal deferred income taxes
Regulatory Liabilities
Regulatory liabilities	276,659	285,221	301,151
Other
Regulatory Liabilities
Regulatory liabilities	$ 18,828	$ 3,409	$ 7,299