Regulatory Assets and Liabilities and Regulatory Matters - Schedule of Regulatory Assets and Liabilities Associated With Gas Utility and Electric Utility (Details) - USD ($) $ in Thousands	Jan. 26, 2018	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017
Regulatory Assets
Regulatory assets		$ 361,638	$ 368,929	$ 394,717
Regulatory Liabilities
Regulatory liabilities		380,721	49,230	50,458
Pension and postretirement benefit obligations		136,687	143,674	179,101
Income taxes recoverable
Regulatory Assets
Regulatory assets		128,267	121,421	120,255
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets		135,263	141,310	175,598
Environmental costs
Regulatory Assets
Regulatory assets		59,788	61,566	62,209
Deferred fuel and power costs
Regulatory Assets
Regulatory assets		738	7,685	1,262
Removal costs, net
Regulatory Assets
Regulatory assets		30,478	30,996	28,840
Other
Regulatory Assets
Regulatory assets		7,104	5,951	6,553
Scenario, Forecast | Subsequent Event | Pennsylvania Public Utilities Commission
Regulatory Assets
Requested operating revenue increase	$ 9,200
Postretirement benefits
Regulatory Liabilities
Regulatory liabilities		17,105	17,493	16,974
Deferred fuel and power refunds
Regulatory Liabilities
Regulatory liabilities		35,278	10,621	13,791
State tax benefits — distribution system repairs
Regulatory Liabilities
Regulatory liabilities		19,888	18,430	16,145
Excess federal deferred income taxes (a)
Regulatory Liabilities
Regulatory liabilities		301,151	0	0
Other
Regulatory Liabilities
Regulatory liabilities		$ 7,299	$ 2,686	$ 3,548