Discontinued Operations and Asset Impairments - Components of Discontinued Operations (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Loss from discontinued operations	$ (0.2)	$ (17.8)	$ (0.4)	$ (105.5)
Benefit for income taxes	0.2	6.1	0.3	38.3
Loss from discontinued operations, net	0.0	(11.7)	(0.1)	(67.2)
TECO Coal [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Revenues	0.0	51.6	0.0	200.4
Loss from operations	(0.1)	(7.4)	(0.2)	(16.4)
Loss on sale	0.0	(10.0)	0.0	(10.0)
Loss on impairment	0.0	0.0	0.0	(78.6)
Loss from discontinued operations	(0.1)	(17.4)	(0.2)	(105.0)
TECO Guatemala [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Loss from operations	(0.1)	(0.4)	(0.2)	(0.5)
Loss from discontinued operations	$ (0.1)	$ (0.4)	$ (0.2)	$ (0.5)