Income Taxes - Schedule of Accumulated Deferred Income Tax Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Deferred income tax liabilities:
Property, plant and equipment	$ 134,414	$ 118,212
Acquisition adjustment	16,790	17,440
Loss on reacquired debt	573	572
Deferred gas costs	607	816
Other	2,850	2,784
Total deferred income tax liabilities	155,234	139,824
Deferred income tax assets:
Pension and other employee benefits	5,390	7,382
Environmental costs	2,083	1,917
Net operating loss carryforwards	1,444	1,587
Self insurance	403	484
Storm reserve liability	1,109	1,058
Other	3,904	2,982
Total deferred income tax assets	14,333	15,410
Deferred Income Taxes Per Consolidated Balance Sheet	$ 140,901	$ 124,414