Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current Income Tax Expense
Federal		$ 4,875	$ 434	$ 4,882
State		1,533	1,311	2,382
Investment tax credit adjustments, net		(23)	(35)	(39)
Total current income tax expense		6,385	1,710	7,225
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	20,520	22,235	14,860
Total Income Tax Expense		26,905	23,945	22,085
Property, plant and equipment
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	21,205	20,382	16,758
Deferred gas costs
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	(1,539)	1,614	(209)
Pensions and other employee benefits
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	(84)	537	(335)
FPU merger related premium cost and deferred gain
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	(556)	(802)	(686)
Net operating loss carryforwards
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	2,078	(112)	62
Other
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	$ (584)	$ 616	$ (730)

[1] Includes $2.1 million, $2.6 million, and $2.1 million of deferred state income taxes for the years 2015, 2014 and 2013, respectively.