Commitments and Contingencies - Additional Information (Detail) $ in Millions				9 Months Ended
	Apr. 05, 2016 USD ($)	Dec. 19, 2013 USD ($)	Feb. 08, 2011 Customer InsuranceCarrier Lawsuit	Sep. 30, 2016 USD ($) Claims	Sep. 30, 2015 USD ($)	Mar. 31, 2011 Customer
Long Term Commitments [Line Items]
Gas supply disruption of high utility customers | Customer			28,700			32,000
Number of class actions settled | Claims				2
Number of insurance carriers | InsuranceCarrier			18
Number of lawsuits for insurance carriers | Lawsuit			2
Dismissed annulment of the award	$ 21.1
Letters of indemnity coal mining permits					$ 93.8
PGS [Member]
Long Term Commitments [Line Items]
Ultimate financial liability to superfund sites and former MGP sites				$ 33.9
PGS [Member] | Tampa Electric Company [Member]
Long Term Commitments [Line Items]
Ultimate financial liability to superfund sites and former MGP sites				$ 33.9
TECO Guatemala Holdings, LLC v. The Republic of Guatemala [Member]
Long Term Commitments [Line Items]
Litigation settlement amount		$ 7.5
Damages awarded		$ 21.1
TECO Guatemala Holdings, LLC v. The Republic of Guatemala [Member] | U.S. prime rate plus [Member]
Long Term Commitments [Line Items]
Litigation settlement interest		2.00%