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	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Regulatory Assets
Regulatory assets		$ 362,842	$ 368,929	$ 392,864
Regulatory Liabilities
Regulatory liabilities		357,482	49,230	58,668
Pension and postretirement benefit obligations		140,224	143,674	181,809
Income taxes recoverable
Regulatory Assets
Regulatory assets		126,509	121,421	117,777
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets		138,287	141,310	179,364
Environmental costs
Regulatory Assets
Regulatory assets		60,760	61,566	61,437
Deferred fuel and power costs
Regulatory Assets
Regulatory assets		108	7,685	0
Removal costs, net
Regulatory Assets
Regulatory assets		31,426	30,996	27,062
Other
Regulatory Assets
Regulatory assets		5,752	5,951	7,224
Scenario, Forecast | Subsequent Event | Pennsylvania Public Utilities Commission
Regulatory Assets
Requested operating revenue increase	$ 9,200
Postretirement benefits
Regulatory Liabilities
Regulatory liabilities		17,315	17,493	17,259
Deferred fuel and power refunds
Regulatory Liabilities
Regulatory liabilities		12,658	10,621	23,809
State tax benefits — distribution system repairs
Regulatory Liabilities
Regulatory liabilities		19,101	18,430	15,579
Excess federal deferred income taxes (a)
Regulatory Liabilities
Regulatory liabilities		303,901	0	0
Other
Regulatory Liabilities
Regulatory liabilities		$ 4,507	$ 2,686	$ 2,021