Income Taxes - Schedule of Accumulated Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred income tax liabilities:
Property, plant and equipment	$ 185,448	$ 152,877
Acquisition adjustment	15,490	16,140
Loss on reacquired debt	485	529
Deferred gas costs	683	2,222
Other	5,961	4,507
Total deferred income tax liabilities	208,067	176,275
Deferred income tax assets:
Pension and other employee benefits	6,570	6,532
Environmental costs	2,445	2,313
Net operating loss carryforwards	943	1,186
Self insurance	278	275
Storm reserve liability	1,153	1,150
Other	4,078	3,755
Total deferred income tax assets	15,467	15,211
Deferred Income Tax Liabilities, Net	$ 192,600	$ 161,064