Discontinued Operations, Assets Held for Sale and Asset Impairments - Components of Discontinued Operations (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations		$ (106.3)	$ (125.4)	$ 5.2
Provision (benefit) for income taxes		(38.6)	(49.4)	(3.8)
Income (loss) from discontinued operations, net		(67.7)	(76.0)	9.0
TECO Coal [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Revenues		200.4	443.6	496.2
Income (loss) from operations		(16.9)	(13.9)	5.4
Loss on impairment		(78.6)	(115.9)	0.0
Loss on sale	$ (10.0)	(10.0)	0.0	0.0
Income (loss) from discontinued operations		(105.5)	(129.8)	5.4
TECO Guatemala [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Income (loss) from operations		(0.8)	4.4	(0.2)
Income (loss) from discontinued operations		(0.8)	4.4	(0.2)
TECO Coal and TECO Guatemala [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Provision (benefit) for income taxes		$ (38.6)	$ (49.4)	$ (3.8)