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Mergers and Acquisitions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions				3 Months Ended		9 Months Ended
	Jul. 12, 2016	Jul. 02, 2016	Jun. 22, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jul. 01, 2016
Business Acquisition [Line Items]
Amount of cash offered for each acquiree common stock share outstanding								$ 27.55
Aggregate purchase price		$ 10,700.0
Senior secured notes								$ 4,200.0
Other expenses yet to be paid				$ 30.0		$ 30.0
Business combination transaction related costs				15.5	$ 15.4	87.0	$ 15.4
Business combination transaction related costs, after tax				9.6		68.1
Investment banking, legal and other consultant costs						27.7
Change-in-control and other compensation payments						42.4
Non-cash SERP curtailment charge						1.3
Contributions from parent				22.0
Accelerated vesting of outstanding stock-based compensation awards				15.5
Business combination transaction related costs payable				20.0		20.0
Dividends payable, declared date			Jun. 22, 2016
Declared a special pro-rated dividend			$ 0.002527
Dividends paid	$ 26.8
Tampa Electric Company [Member] \| Emera Inc. [Member]
Business Acquisition [Line Items]
Amount of cash offered for each acquiree common stock share outstanding								$ 27.55
Aggregate purchase price		$ 10,700.0
Senior secured notes								$ 4,200.0
Senior secured notes								2,300.0
Regulated Gas Revenues [Member]
Business Acquisition [Line Items]
Recognition of bill credit				8.0		8.0
Pipeline [Member]
Business Acquisition [Line Items]
Other commitment								5.0
Natural Gas Infrastructure [Member]
Business Acquisition [Line Items]
Other commitment								10.0
Annual Commitment to Charitable Contributions and Certain Enterprises [Member]
Business Acquisition [Line Items]
Other commitment								0.8
Period of commitment		3 years
Annual Bill Reduction Credit [Member]
Business Acquisition [Line Items]
Other commitment								4.0
Economic Development Projects [Member]
Business Acquisition [Line Items]
Other commitment								$ 5.0
Period of commitment		5 years
Commitments to NMPRC, Pretax [Member]
Business Acquisition [Line Items]
Other commitment				30.4		30.4
Commitments to NMPRC, After Tax [Member]
Business Acquisition [Line Items]
Other commitment				$ 17.7		$ 17.7