Derivative Instruments - Additional Information (Detail) Mcf in Thousands, $ in Thousands, gal in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) $ / gal gal Mcf	Dec. 31, 2017 USD ($) gal
Derivative [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	$ 1,288	$ 2,391
Energy Marketing Contracts Assets, Current	534	1,286
Energy Marketing Contract Liabilities, Current	(886)	(6,247)
Accounts Payable Subject To Master Netting Arrangement	$ 1,288	2,391
Natural Gas Futures [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | Mcf	500
Hedging Liability [Member] | Natural Gas Futures [Member]
Derivative [Line Items]
Energy Marketing Contract Liabilities, Current	$ (779)	(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] | Natural Gas Futures [Member]
Derivative [Line Items]
Energy Marketing Contract Liabilities, Current	(120)
Designated as Hedging Instrument [Member] | Mark To Market Energy Assets [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	324	1,181
Energy Marketing Contract Liabilities, Current	(30)	0
Designated as Hedging Instrument [Member] | Mark-to-market energy liabilities [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	$ 18
Not Designated as Hedging Instrument [Member] | Natural Gas Futures [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | Mcf	1,100
Not Designated as Hedging Instrument [Member] | Mark To Market Energy Assets [Member] | Natural Gas Futures [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	$ 90
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	$ (77)	$ (5,776)
PESCO [Member] | Natural Gas Futures [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | Mcf	16,900
2017 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	1.4
2017 [Member] | Natural Gas Swaps [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | Mcf	844
2017 [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal		7.7
Derivative, Cash Received on Hedge	$ 11
2017 [Member] | Strike Price 4 [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Strike Price Per Gallon For The Propane Swap Agreements | $ / gal	0.5900
2017 [Member] | Designated as Hedging Instrument [Member] | Mark To Market Energy Assets [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Energy Marketing Contracts Assets, Current	$ 306
2018 [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	1.4
2018 [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Derivative, Cash Received on Hedge	$ 645
2018 [Member] | Strike Price Minimum [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Strike Price Per Gallon For The Propane Swap Agreements | $ / gal	0.76
2018 [Member] | Strike Price 2 [Member] | Propane Swap Agreement [Member]
Derivative [Line Items]
Strike Price Per Gallon For The Propane Swap Agreements | $ / gal	0.875
Gross [Member]
Derivative [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	$ 5,723	$ 8,283
Accounts Payable Subject To Master Netting Arrangement	10,326	16,643
Net [Member]
Derivative [Line Items]
Accounts Receivable Subject to Master Netting Arrangement	4,435	5,892
Accounts Payable Subject To Master Netting Arrangement	$ 9,038	$ 14,252