Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current Income Tax Expense
Federal		$ (4,898)	$ 4,875	$ 434
State		2,053	1,533	1,311
Other		(71)	(23)	(35)
Total current income tax expense		(2,916)	6,385	1,710
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	31,257	20,520	22,235
Total Income Tax Expense		28,341	26,905	23,945
Property, plant and equipment
Deferred Income Tax Expense (1)
Total deferred income tax expense		31,062	21,205	20,382
Deferred gas costs
Deferred Income Tax Expense (1)
Total deferred income tax expense		1,163	(1,539)	1,614
Pensions and other employee benefits
Deferred Income Tax Expense (1)
Total deferred income tax expense		237	(84)	537
FPU merger related premium cost and deferred gain
Deferred Income Tax Expense (1)
Total deferred income tax expense		(572)	(556)	(802)
Net operating loss carryforwards
Deferred Income Tax Expense (1)
Total deferred income tax expense		(9)	2,078	(112)
Other
Deferred Income Tax Expense (1)
Total deferred income tax expense		$ (624)	$ (584)	$ 616

[1] (1)Includes $2.1 million, $2.1 million and $2.6 million of deferred state income taxes for the years 2016, 2015 and 2014, respectively.