Variable Interest Entities - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) MW	Sep. 30, 2014 USD ($)
Long Term Contract For Purchase Of Electric Power [Line Items]
Purchased power	$ 23.8	$ 21.0	$ 60.5	$ 59.1
Tampa Electric Company [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Purchased power	23.8	21.0	60.5	59.1
Power Purchase Agreements [Member] | Variable Interest Entities [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Purchased power	10.7	8.1	26.0	20.9
Power Purchase Agreements [Member] | Variable Interest Entities [Member] | Tampa Electric Company [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Purchased power	$ 10.7	$ 8.1	$ 26.0	$ 20.9
Minimum [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range | MW			117
Minimum [Member] | Tampa Electric Company [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range | MW			117
Maximum [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range | MW			157
Maximum [Member] | Tampa Electric Company [Member]
Long Term Contract For Purchase Of Electric Power [Line Items]
Multiple PPAs range | MW			157