Mergers and Acquisitions - Additional Information (Detail) - USD ($)			3 Months Ended		9 Months Ended
	Sep. 04, 2015	Sep. 02, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Business Acquisition [Line Items]
Termination fee payable by company	$ 212,500,000
Business combination transaction related costs			$ 15,400,000
Obligations associated with severance benefits costs			300,000		$ 300,000		$ 2,600,000
Tampa Electric Company [Member]
Business Acquisition [Line Items]
Termination fee payable by company	$ 212,500,000
Emera Inc. [Member]
Business Acquisition [Line Items]
Amount of cash offered for each acquiree common stock share outstanding	$ 27.55
Aggregate purchase price	$ 10,400,000,000
Senior secured notes	$ 3,900,000,000
Merger agreement termination extension regulatory approval period	6 months
Termination fee payable by acquirer	$ 326,900,000
Emera Inc. [Member] | Tampa Electric Company [Member]
Business Acquisition [Line Items]
Amount of cash offered for each acquiree common stock share outstanding	$ 27.55
Aggregate purchase price	$ 10,400,000,000
Senior secured notes	$ 3,900,000,000
Merger agreement termination extension regulatory approval period	6 months
Termination fee payable by acquirer	$ 326,900,000
Senior secured notes	$ 2,300,000,000
NMGI [Member]
Business Acquisition [Line Items]
Aggregate purchase price		$ 950,000,000
Business combination transaction related costs			200,000	$ 900,000	1,200,000	$ 5,700,000
Business acquisition, effective date		Sep. 02, 2014
After-tax non-recurring acquisition consummation, integration and other costs				900,000		5,700,000
New Mexico Gas Company [Member]
Business Acquisition [Line Items]
Senior secured notes		$ 200,000,000
NMGI and NMGC [Member]
Business Acquisition [Line Items]
Revenue			43,700,000	16,200,000	216,700,000	16,200,000
Net income			$ (3,900,000)	$ (2,000,000)	$ 7,600,000	$ (2,000,000)