Income Taxes - Schedule of Accumulated Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred income tax liabilities:
Property, plant and equipment	$ 218,074	$ 185,448
Acquisition adjustment	14,840	15,490
Loss on reacquired debt	442	485
Deferred gas costs	1,846	683
Other	6,375	5,961
Total deferred income tax liabilities	241,577	208,067
Deferred income tax assets:
Pension and other employee benefits	6,230	6,570
Environmental costs	2,592	2,445
Net operating loss carryforwards	952	943
Investment tax credit carryforwards	2,643	0
Self insurance	189	278
Storm reserve liability	1,131	1,153
Other	4,946	4,078
Total deferred income tax assets	18,683	15,467
Deferred Income Tax Liabilities, Net	$ 222,894	$ 192,600