Discontinued Operations, Assets Held for Sale and Asset Impairments Sale (Carrying Amount of Assets and Liabilities Held for Sale) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Current assets	$ 133.7	$ 0
Property, plant and equipment, net and other long-term assets	78.6	0
Current liabilities	41.1	0
Long-term liabilities	64.4	0
TECO Coal [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Current assets	133.7
Property, plant and equipment, net and other long-term assets	78.6
Total assets held for sale	212.3
Current liabilities	41.1
Long-term liabilities	64.4
Total liabilities associated with assets held for sale	$ 105.5