Mergers and Acquisitions - Additional Information (Detail)			12 Months Ended
	Sep. 04, 2015 USD ($) $ / shares	Sep. 02, 2014 USD ($) Employee Customer Country Pipelines	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Aggregate purchase price		$ 950,000,000
Termination fee payable by company	$ 212,500,000
Business combination incremental transaction related costs			$ 17,000,000
Customer percentage of state population		60.00%
Mileage of transmission pipeline served | Pipelines		1,600
Mileage of distribution lines served | Pipelines		10,000
After-tax non-recurring acquisition consummation, integration and other costs				$ 8,600,000	$ 6,200,000
Obligations associated with severance benefits costs			700,000	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 | $ / shares	$ 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 | $ / shares	$ 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 acquisition, effective date		Sep. 02, 2014
NMGC [Member]
Business Acquisition [Line Items]
Senior secured notes		$ 200,000,000
Number of employees | Employee		720
Minimum number of customers served | Customer		513,000
Customer rate credit		$ 2,000,000
Increase in rate credit		$ 4,000,000
Goodwill expected tax deductible amount			146,100,000
NM [Member]
Business Acquisition [Line Items]
Number of counties in which entity operates | Country		23
Number of counties | Country		33
NMGI and NMGC [Member]
Business Acquisition [Line Items]
Senior secured notes		$ 22,700,000
Revenue			316,500,000	137,500,000
Net income			$ 19,600,000	$ 8,200,000