Employee Postretirement benefits - Schedule of Pension Plan Assets (Detail) - TECO Energy [Member]	Dec. 31, 2019	Dec. 31, 2018
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	58.00%	46.00%
Actual Asset Allocation [Member] | Fixed Income Securities [Member]
Asset Category
Actual Allocation, End of Year	42.00%	54.00%
Target Allocation [Member]
Asset Category
2019 Target Allocation	100.00%	100.00%
Target Allocation [Member] | Equity Securities [Member] | Minimum [Member]
Asset Category
2019 Target Allocation	57.00%	47.00%
Target Allocation [Member] | Equity Securities [Member] | Maximum [Member]
Asset Category
2019 Target Allocation	63.00%	53.00%
Target Allocation [Member] | Fixed Income Securities [Member] | Minimum [Member]
Asset Category
2019 Target Allocation	37.00%	47.00%
Target Allocation [Member] | Fixed Income Securities [Member] | Maximum [Member]
Asset Category
2019 Target Allocation	43.00%	53.00%