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Employee Postretirement Benefits - Schedule of Benefit Payments (Detail) - TECO Energy [Member]
$ in Millions
Dec. 31, 2018
USD ($)
Pension Benefits [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected Benefit Payments - 2019 $ 57
Expected Benefit Payments - 2020 55
Expected Benefit Payments - 2021 59
Expected Benefit Payments - 2022 60
Expected Benefit Payments - 2023 60
Expected Benefit Payments - 2024 - 2028 333
Other Postretirement Benefits [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Expected Benefit Payments - 2019 12
Expected Benefit Payments - 2020 12
Expected Benefit Payments - 2021 12
Expected Benefit Payments - 2022 12
Expected Benefit Payments - 2023 12
Expected Benefit Payments - 2024 - 2028 $ 59