Discontinued Operations and Asset Impairments - Components of Discontinued Operations (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Loss from operations	$ (7.4)	$ (0.4)	$ (16.4)	$ (4.2)
Loss from discontinued operations	(17.8)	(98.8)	(105.5)	(97.6)
Benefit from income taxes	(6.1)	(36.9)	(38.3)	(38.2)
Loss on discontinued operations, net	(11.7)	(61.9)	(67.2)	(59.4)
TECO Coal [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Revenues	51.6	101.6	200.4	328.3
Loss on sale	(10.0)	0.0	(10.0)	0.0
Loss on impairment	0.0	(98.4)	(78.6)	(98.4)
Loss from discontinued operations	(17.4)	(98.8)	(105.0)	(102.6)
TECO Guatemala [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Loss from operations	(0.4)	0.0	(0.5)	5.0
Loss from discontinued operations	(0.4)	0.0	(0.5)	5.0
TECO Coal and TECO Guatemala [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Benefit from income taxes	$ (6.1)	$ (36.9)	$ (38.3)	$ (38.2)