Discontinued Operations and Asset Impairments - Additional Information (Detail) - TECO Coal [Member] - USD ($) $ in Millions	3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Sep. 21, 2015
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Contingent Payments								$ 60.0
Loss on sale, pretax	$ (10.0)		$ 0.0		$ (10.0)	$ 0.0
Settlement charge, related to unfunded black lung obligations					$ 7.7
Transaction costs, pretax				$ 12.3
Legal and other consultant costs				2.5
Severance and other employee costs				$ 9.8
Total pretax impairment charges		$ 78.6					$ 115.9