Income Taxes - Schedule of Income Tax Expense (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current Income Tax Expense
Federal		$ (845)	$ 2,803	$ (4,898)
State		660	492	2,053
Other		(47)	(71)	(71)
Total current income tax expense		(232)	3,224	(2,916)
Deferred Income Tax Expense (1)
Total deferred income tax expense	[1]	21,226	11,085	31,257
Total Income Tax Expense		20,994	14,309	28,341
Property, plant and equipment
Deferred Income Tax Expense (1)
Total deferred income tax expense		19,164	8,314	31,062
Deferred gas costs
Deferred Income Tax Expense (1)
Total deferred income tax expense		(1,435)	2,002	1,163
Pensions and other employee benefits
Deferred Income Tax Expense (1)
Total deferred income tax expense		463	180	237
FPU merger related premium cost and deferred gain
Deferred Income Tax Expense (1)
Total deferred income tax expense		(528)	(1,148)	(572)
Net operating loss carryforwards
Deferred Income Tax Expense (1)
Total deferred income tax expense		(331)	193	(9)
Other
Deferred Income Tax Expense (1)
Total deferred income tax expense		$ 3,893	$ 1,544	$ (624)

[1] (1) Includes $3.5 million, $873,000 and $2.1 million of deferred state income taxes for the years 2018, 2017 and 2016, respectively.