Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	1 Months Ended		12 Months Ended
	Sep. 30, 2011	Jul. 31, 2011	Dec. 31, 2012 Y	Dec. 31, 2011	Dec. 31, 2010
Summary Of Accounting Policies [Line Items]
Non-refundable contributions for construction received			$ 1,100,000	$ 286,000.0
Allowed funds used during construction			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
Accumulated depreciation, plant in service assets under operating lease			291,000	218,000
Operating lease contract date			Jan. 01, 2009
Net property, plant and equipment			541,781,000	487,704,000
Accumulated depreciation			155,378,000	137,784,000
Proceeds from sale of Internet Protocol address asset		553,000
Non-operating pre-tax gain on sale of Internet Protocol address asset		553,000	(93,000)	453,000	(113,000)
Agreement for sale of office building	2,200,000		2,279,000	937,000	113,000
Purchase price allocated to parcel			600,000
Depreciation and accretion reported in operations expenses			5,500,000	5,100,000	4,400,000
Deferred revenue refund payment, period			1 year
Investment with an original maturity period			Three months or less
Pension benefit [Member]
Summary Of Accounting Policies [Line Items]
Percentage of change in discount rate			0.25%
Estimated increase in annual pension and postretirement costs			11,000
Estimated decrease in annual pension and postretirement costs			13,000
Pensions and other employee benefits [Member]
Summary Of Accounting Policies [Line Items]
Percentage of change in rate of return			0.25%
Estimated change in annual post retirement and supplemental executive retirement plans			124,000
Natural gas transmission operation [Member]
Summary Of Accounting Policies [Line Items]
Net property, plant and equipment			6,700,000
Length of pipeline			16
Ownership interest			45.00%
Accumulated depreciation			$ 28,000