Employee Postretirement benefits - Schedule of Pension Plan Assets (Detail) - TECO Energy [Member]	Dec. 31, 2018	Dec. 31, 2017
Actual Asset Allocation [Member]
Asset Category
Actual Allocation, End of Year	100.00%	100.00%
Actual Asset Allocation [Member] | Equity Securities [Member]
Asset Category
Actual Allocation, End of Year	46.00%	51.00%
Actual Asset Allocation [Member] | Fixed Income Securities [Member]
Asset Category
Actual Allocation, End of Year	54.00%	49.00%
Target Allocation [Member]
Asset Category
2018 Target Allocation	100.00%
Target Allocation [Member] | Equity Securities [Member] | Minimum [Member]
Asset Category
2018 Target Allocation	47.00%
Target Allocation [Member] | Equity Securities [Member] | Maximum [Member]
Asset Category
2018 Target Allocation	53.00%
Target Allocation [Member] | Fixed Income Securities [Member] | Minimum [Member]
Asset Category
2018 Target Allocation	47.00%
Target Allocation [Member] | Fixed Income Securities [Member] | Maximum [Member]
Asset Category
2018 Target Allocation	53.00%