Discontinued Operations, Assets Held for Sale and Asset Impairments - Carrying Amount of Assets and Liabilities Held for Sale (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Current assets	$ 97.1		$ 109.6
Property, plant and equipment, net and other long-term assets	60.9		59.8
Current liabilities	33.7		39.4
Long-term liabilities	65.7		65.4
TECO Coal [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Current assets	97.1	[1]	109.6	[1]
Property, plant and equipment, net and other long-term assets	60.9		59.8
Total assets held for sale	158.0		169.4
Current liabilities	33.7		39.4
Long-term liabilities	65.7		65.4
Total liabilities associated with assets held for sale	$ 99.4		$ 104.8

[1] (1) Certain items within Current assets are excluded from the SPA working capital adjustments.