Income Taxes - Schedule of Accumulated Deferred Income Tax Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred income tax liabilities:
Property, plant and equipment	$ 118,212	$ 105,850
Acquisition adjustment	17,440	18,090
Deferred gas costs	816	301
Loss on reacquired debt	572	608
Other	2,784	3,872
Total deferred income tax liabilities	139,824	128,721
Deferred income tax assets:
Pension and other employee benefits	7,382	7,796
Environmental costs	1,917	1,835
Net operating loss carryforwards	1,587	2,401
Self insurance	484	452
Storm reserve liability	1,058	1,085
Other	2,982	2,240
Total deferred income tax assets	15,410	15,809
Deferred Income Taxes Per Consolidated Balance Sheet	$ 124,414	$ 112,912