Rates and Other Regulatory Activities - Additional Information (Detail) (USD $)	12 Months Ended		1 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2012 DTH	May 14, 2012 Eastern Shore Mainline Expansion Project [Member]	Jan. 30, 2012 Florida [Member]	Nov. 30, 2009 Florida [Member]	Dec. 31, 2012 Florida [Member] Customer mi	Dec. 31, 2011 Florida [Member]	Dec. 31, 2012 Eastern Shore Gas Company [Member] DTH	Dec. 31, 2011 Eastern Shore Gas Company [Member]	Dec. 31, 2012 Eastern Shore Gas Company [Member] Eastern Shore Mainline Expansion Project [Member] DTH	May 14, 2012 Eastern Shore Gas Company [Member] Eastern Shore Mainline Expansion Project [Member]
Investment In Affiliates [Line Items]
Inclusion of acquisition adjustment			$ 34,200,000
Costs related to mergers			2,200,000
Amortization period of merger cost			30 years	5 years
Regulatory asset which represents portion of the goodwill allowed to be recovered in future rates after the effective date of the Florida PSC order						31,700,000
Regulatory asset related to the gross-up of the acquisition adjustment for income tax					349,000	18,100,000
Portion of the merger-related costs allowed to be recovered in future rates after the effective date						1,300,000
Amortization expense in depreciation and amortization in the accompanying of income condensed consolidated statement					2,400,000
Amortization expense related to assets					2,400,000
Net of tax year one					1,400,000
Amortization of acquired intangible assets year two					2,300,000
Net of tax year two					1,400,000
Amortization of acquired intangible assets year three					1,800,000
Net of tax year three					1,100,000
Expected percentage owned pipeline under the agreement					45.00%
Joint construction, ownership and operation of a pipeline extension					16
Estimated project cost of pipeline					5,800,000
Annual charge of new jointly-owned pipeline					800,000
Annual charge to provide transmission service					2,100,000
Period to replace qualifying mains and services	10 years				10 years
Duration of right to give notice to FPU					180 days
Duration of closing of the purchase					12 months
Loss of earnings generated by FPU from number of customers					3,000
Period of current agreement exceeds					2 years
Expiration date					Dec. 31, 2019
Amortization period of regulatory asset					Five
Litigation expense					1,400,000
Cost incurred	75,000,000
Reduction in depreciation due to change in depreciation rates					227,000
Regulatory liabilities contingency gain					1,900,000
Tax contingency gain on regulatory liabilities					1,200,000
Regulatory liabilities gross up tax					748,000
Annual cost of service							29,100,000
Pre-tax return							13.90%
Number of dekatherms per day							15,000
Regulatory liability related to the refund due to customers as a result of the settlement								1,300,000
Approval for volume of additional firm service to construct own and operate the facilities									15,040
Estimated capital cost of Mainline Expansion Project		16,300,000
Extension requested by protesting non-profit organization for public review and comment										60 days
Additional firm transportation service capacity	17,500
Estimated capital costs of project							12,100,000
Filing period for fuel retention percentage and cash out surcharge							24 months
Amount of refund to eligible customers	$ 320,000