Regulatory Assets and Liabilities and Regulatory Matters - Schedule of Regulatory Assets and Liabilities Associated With Gas Utility and Electric Utility (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Regulatory Assets
Regulatory assets	$ 298,763	$ 301,002	$ 362,842
Regulatory Liabilities
Regulatory liabilities	373,468	390,175	357,482
Income taxes recoverable
Regulatory Assets
Regulatory assets	115,204	110,129	126,509
Underfunded pension and postretirement plans
Regulatory Assets
Regulatory assets	85,334	87,106	138,287
Environmental costs
Regulatory Assets
Regulatory assets	58,461	58,836	60,760
Removal costs, net
Regulatory Assets
Regulatory assets	31,332	32,025	31,426
Other
Regulatory Assets
Regulatory assets	8,432	12,906	5,860
Postretirement benefits
Regulatory Liabilities
Regulatory liabilities	17,315	17,781	17,315
Deferred fuel and power refunds
Regulatory Liabilities
Regulatory liabilities	22,203	36,723	12,658
State tax benefits — distribution system repairs
Regulatory Liabilities
Regulatory liabilities	23,453	22,611	19,101
PAPUC temporary rates order
Regulatory Liabilities
Regulatory liabilities	24,766	24,430	0
Excess federal deferred income taxes
Regulatory Liabilities
Regulatory liabilities	280,940	285,221	303,901
Other
Regulatory Liabilities
Regulatory liabilities	$ 4,791	$ 3,409	$ 4,507