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Employee Postretirement Benefits - Schedule of Benefit Payments (Detail) - TECO Energy [Member]
$ in Millions
Dec. 31, 2017
USD ($)
Pension Benefits [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected Benefit Payments - 2018 $ 58
Expected Benefit Payments - 2019 56
Expected Benefit Payments - 2020 56
Expected Benefit Payments - 2021 59
Expected Benefit Payments - 2022 61
Expected Benefit Payments - 2023 - 2027 323
Other Postretirement Benefits [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected Benefit Payments - 2018 12
Expected Benefit Payments - 2019 12
Expected Benefit Payments - 2020 12
Expected Benefit Payments - 2021 12
Expected Benefit Payments - 2022 12
Expected Benefit Payments - 2023 - 2027 $ 61