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Employee Postretirement Benefits - Schedule of Benefit Payments (Detail) - TECO Energy [Member]
$ in Millions
Dec. 31, 2016
USD ($)
Pension Benefits [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected Benefit Payments - 2017 $ 78.3
Expected Benefit Payments - 2018 51.8
Expected Benefit Payments - 2019 55.6
Expected Benefit Payments - 2020 56.1
Expected Benefit Payments - 2021 58.7
Expected Benefit Payments - 2022 - 2026 312.4
Other Postretirement Benefits [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected Benefit Payments - 2017 11.0
Expected Benefit Payments - 2018 11.2
Expected Benefit Payments - 2019 11.5
Expected Benefit Payments - 2020 11.6
Expected Benefit Payments - 2021 11.7
Expected Benefit Payments - 2022 - 2026 $ 58.9