Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended
	Jul. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jul. 31, 2011 Pension benefit	Dec. 31, 2013 Pensions and other employee benefits	Dec. 31, 2013 Natural Gas Operations mi	Dec. 31, 2013 Natural Gas Operations Pension benefit	Dec. 31, 2012 Natural Gas Operations Pension benefit	Dec. 31, 2010 Parent	Dec. 31, 2009 Parent
Summary Of Accounting Policies [Line Items]
Premium paid for acquisition										$ 746,000	$ 34,200,000
Non-refundable contributions for construction received		785,000	1,100,000
Allowed funds used during construction		131,000	111,000	25,000
Property, plant and equipment in service assets under operating lease contract		1,400,000
Annual revenue from operating lease		264,000
Operating lease term		20 years
Accumulated depreciation, plant in service assets under operating lease		291,000	363,000
Capital Leased Assets, Gross		7,000,000
Operating lease contract date		Jan. 01, 2009
Net property, plant and equipment		631,246,000	541,781,000				6,700,000
Length of pipeline							16
Ownership interest							4500.00%
Accumulated depreciation		174,148,000	155,378,000					361,000	28,000
Proceeds from sale of Internet Protocol address asset					553,000
Non-operating pre-tax gain on sale of Internet Protocol address asset	553,000	152,000	(93,000)	453,000
Depreciation and accretion reported in operations expenses		6,100,000	5,500,000	5,100,000
Deferred revenue refund payment, period		1 year
Investment with an original maturity period		Three months or less
Percentage of change in discount rate						0.25%
Percentage of change in rate of return						0.25%
Estimated increase in annual pension and postretirement costs						4,000
Estimated decrease in annual pension and postretirement costs						40,000
Estimated change in annual post retirement and supplemental executive retirement plans						$ 132,000