Income Taxes - Summary of Reconciliation of Statutory Federal Tax and Effective Income Tax Rates (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Reconciliation of Effective Income Tax Rates Continuing Operations
Federal income tax expense (2)	$ 19,205	[1]	$ 16,745	[1]	$ 16,146	[1]
State income taxes, net of federal benefit	3,105		2,571		2,216
ESOP dividend deduction	(256)		(235)		(236)
Other	31		215		(137)
Total Income Tax Expense	$ 22,085		$ 19,296		$ 17,989
Effective Income Tax Rate	40.25%		40.07%		39.44%

[1] As of December 31, 2013 2012(in thousands) Deferred Income Taxes Deferred income tax liabilities: Property, plant and equipment$134,414 $118,212Acquisition adjustment16,790 17,440Loss on reacquired debt573 572Deferred gas costs607 816Other2,850 2,784Total deferred income tax liabilities155,234 139,824Deferred income tax assets: Pension and other employee benefits5,390 7,382Environmental costs2,083 1,917Net operating loss carryforwards1,444 1,587Self insurance403 484Storm reserve liability1,109 1,058Other3,904 2,982Total deferred income tax assets14,333 15,410Deferred Income Taxes Per Consolidated Balance Sheet$140,901 $124,414