Income Taxes - Schedule of Accumulated Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2016
Disclosure Income Taxes Schedule Of Accumulated Deferred Income Tax Assets And Liabilities [Abstract]
Deferred State and Local Income Tax Expense (Benefit)	$ 900	$ 2,100
Deferred income tax liabilities:
Property, plant and equipment	133,581		$ 218,074
Acquisition adjustment	9,323		14,840
Loss on reacquired debt	153		442
Deferred gas costs	2,574		1,846
Other	5,422		6,375
Total deferred income tax liabilities	151,053		241,577
Deferred income tax assets:
Pension and other employee benefits	4,698		6,230
Environmental costs	1,744		2,592
Net operating loss carryforwards	1,625		952
Investment tax credit carryforwards	0		2,643
Self insurance	164		189
Storm reserve liability	717		1,131
Other	6,255		4,946
Total deferred income tax assets	15,203		18,683
Deferred Income Tax Liabilities, Net	$ 135,850		$ 222,894