Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current Income Tax Expense
Federal		$ 2,803	$ (4,898)	$ 4,875
State		492	2,053	1,533
Other		(71)	(71)	(23)
Total current income tax expense		3,224	(2,916)	6,385
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	11,085	31,257	20,520
Total Income Tax Expense		14,309	28,341	26,905
Property, plant and equipment
Deferred Income Tax Expense (1)
Total deferred income tax expense		8,314	31,062	21,205
Deferred gas costs
Deferred Income Tax Expense (1)
Total deferred income tax expense		2,002	1,163	(1,539)
Pensions and other employee benefits
Deferred Income Tax Expense (1)
Total deferred income tax expense		180	237	(84)
FPU merger related premium cost and deferred gain
Deferred Income Tax Expense (1)
Total deferred income tax expense		(1,148)	(572)	(556)
Net operating loss carryforwards
Deferred Income Tax Expense (1)
Total deferred income tax expense		193	(9)	2,078
Other
Deferred Income Tax Expense (1)
Total deferred income tax expense		$ 1,544	$ (624)	$ (584)

[1] (1)Includes $873,000, $2.1 million and $2.1 million of deferred state income taxes for the years 2017, 2016 and 2015, respectively.