Derivative Instruments - Additional Information (Detail) $ in Thousands, Mcf in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) Mcf	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Derivative, Amount of Hedged Item	$ 290	$ 0
Other Payables to Broker-Dealers and Clearing Organizations	18	0
Other Receivables from Broker-Dealers and Clearing Organizations	2,060	6,300
Accounts Receivable Subject to Master Netting Arrangement	1,158	2,391
Energy Marketing Contracts Assets, Current	10,568	1,286
Energy Marketing Contract Liabilities, Current	(9,774)	(6,247)
Accounts Payable Subject To Master Netting Arrangement	1,158	2,391
Put Option [Member]
Derivative [Line Items]
Derivative, Cost of Hedge	28
Hedging Liability [Member] | Natural Gas Futures [Member]
Derivative [Line Items]
Energy Marketing Contract Liabilities, Current	(8,168)	(469)
Hedging Liability [Member] | Natural Gas Swaps [Member]
Derivative [Line Items]
Energy Marketing Contract Liabilities, Current	0	(2)
Designated as Hedging Instrument [Member] | Mark To Market Energy Assets [Member] | Put Option [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	19	0
Designated as Hedging Instrument [Member] | Mark To Market Energy Assets [Member] | Natural Gas Futures [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	8,114	92
Designated as Hedging Instrument [Member] | Mark To Market Energy Assets [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	594	1,181
Designated as Hedging Instrument [Member] | Mark-to-market energy liabilities [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Energy Marketing Contract Liabilities, Current	$ (4)	0
Not Designated as Hedging Instrument [Member] | Natural Gas Futures [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | Mcf	1.1
Not Designated as Hedging Instrument [Member] | Mark To Market Energy Assets [Member] | Natural Gas Futures [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	$ 1,841
Not Designated as Hedging Instrument [Member] | Mark To Market Energy Assets [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	0	13
Not Designated as Hedging Instrument [Member] | Mark-to-market energy liabilities [Member] | Natural Gas Futures [Member]
Derivative [Line Items]
Energy Marketing Contract Liabilities, Current	$ (1,602)	(5,776)
PESCO [Member] | Natural Gas Futures [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | Mcf	22.1
Gross [Member]
Derivative [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	$ 7,042	8,283
Accounts Payable Subject To Master Netting Arrangement	11,264	16,643
Net [Member]
Derivative [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	5,884	5,892
Accounts Payable Subject To Master Netting Arrangement	$ 10,106	$ 14,252