Income Taxes - Schedule of Income Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current Income Tax Expense
Federal	$ 434us-gaap_CurrentFederalTaxExpenseBenefit	$ 4,882us-gaap_CurrentFederalTaxExpenseBenefit	$ 3,483us-gaap_CurrentFederalTaxExpenseBenefit
State	1,311	2,382	1,990
Investment tax credit adjustments, net	(35)us-gaap_IncomeTaxCreditsAndAdjustments	(39)us-gaap_IncomeTaxCreditsAndAdjustments	(58)us-gaap_IncomeTaxCreditsAndAdjustments
Total current income tax expense	1,710us-gaap_CurrentIncomeTaxExpenseBenefit	7,225us-gaap_CurrentIncomeTaxExpenseBenefit	5,415us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred Income Tax Expense (1)
Total deferred income tax expense	22,235us-gaap_DeferredIncomeTaxExpenseBenefit	14,860us-gaap_DeferredIncomeTaxExpenseBenefit	13,881us-gaap_DeferredIncomeTaxExpenseBenefit
Total Income Tax Expense	23,945us-gaap_IncomeTaxExpenseBenefit	22,085us-gaap_IncomeTaxExpenseBenefit	19,296us-gaap_IncomeTaxExpenseBenefit
Property, plant and equipment
Deferred Income Tax Expense (1)
Total deferred income tax expense	20,382	16,758	13,688
Deferred gas costs
Deferred Income Tax Expense (1)
Total deferred income tax expense	1,614	(209)	515
Pensions and other employee benefits
Deferred Income Tax Expense (1)
Total deferred income tax expense	537	(335)	553
FPU merger related premium cost and deferred gain
Deferred Income Tax Expense (1)
Total deferred income tax expense	(802)	(686)	(509)
Net operating loss carryforwards
Deferred Income Tax Expense (1)
Total deferred income tax expense	(112)	62	740
Other
Deferred Income Tax Expense (1)
Total deferred income tax expense	$ 616	$ (730)	$ (1,106)

Amount of current federal tax expense (benefit) pertaining to income (loss) from continuing operations.

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Amount of current state and local tax expense (benefit) pertaining to income (loss) from continuing operations.

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A credit or adjustment for government or taxing authority authorized decrease in taxes owed as a result of meeting certain tax policy conditions.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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