Commitments and Contingencies - Additional Information (Detail) $ in Millions					3 Months Ended
	Apr. 18, 2016 USD ($)	Apr. 05, 2016 USD ($)	Dec. 19, 2013 USD ($)	Feb. 08, 2011 Customer InsuranceCarrier Lawsuit	Mar. 31, 2016 USD ($) 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
Letters of indemnity coal mining permits						$ 93.8
PGS [Member]
Long Term Commitments [Line Items]
Total potential penalties					$ 3.9
Settlement agreement date					Apr. 18, 2016
Ultimate financial liability to superfund sites and former MGP sites					$ 33.9
PGS [Member] | Tampa Electric Company [Member]
Long Term Commitments [Line Items]
Total potential penalties					$ 3.9
Settlement agreement date					Apr. 18, 2016
Ultimate financial liability to superfund sites and former MGP sites					$ 33.9
FPSC [Member]
Long Term Commitments [Line Items]
Settlement agreement date					May 05, 2016
FPSC [Member] | Tampa Electric Company [Member]
Long Term Commitments [Line Items]
Settlement agreement date					May 05, 2016
Subsequent Event [Member]
Long Term Commitments [Line Items]
Dismissed annulment of the award		$ 21.1
Subsequent Event [Member] | PGS [Member]
Long Term Commitments [Line Items]
Civil penalty amount	$ 1.0
Customer refunds	2.0
Subsequent Event [Member] | PGS [Member] | Tampa Electric Company [Member]
Long Term Commitments [Line Items]
Civil penalty amount	1.0
Customer refunds	$ 2.0
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%