Discontinued Operations and Asset Impairments - Carrying Amount of Assets and Liabilities Held for Sale (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Current assets	$ 0.0	$ 109.6
Property, plant and equipment, net and other long-term assets	0.0	59.8
Current liabilities	0.0	39.4
Long-term liabilities	0.0	65.4
TECO Coal [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Current assets	0.0	109.6
Property, plant and equipment, net and other long-term assets	0.0	59.8
Total assets held for sale	0.0	169.4
Current liabilities	0.0	39.4
Long-term liabilities	0.0	65.4
Total liabilities associated with assets held for sale	$ 0.0	$ 104.8