Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Regulatory assets:
Regulatory assets	$ 480.5	$ 440.0
Less: Current portion	21.8	44.8
Long-term regulatory assets	458.7	395.2
Regulatory liabilities:
Regulatory liabilities	867.0	800.6
Less: Current portion	148.2	84.8
Long-term regulatory liabilities	718.8	715.8
Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	458.8	418.1
Less: Current portion	20.6	44.3
Long-term regulatory assets	438.2	373.8
Regulatory liabilities:
Regulatory liabilities	737.4	686.7
Less: Current portion	140.4	83.2
Long-term regulatory liabilities	597.0	603.5
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	83.4	74.7
Regulatory Tax Asset [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	83.1	74.6
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory assets:
Regulatory assets	4.5	5.5
Cost-Recovery Clauses - Deferred Balances [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	4.4	5.2
Cost-Recovery Clauses - Offsets to Derivative Liabilities [Member]
Regulatory assets:
Regulatory assets	2.1	26.5
Cost-Recovery Clauses - Offsets to Derivative Liabilities [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	1.5	26.2
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	54.8	54.0
Environmental Remediation [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	54.8	54.0
Postretirement Benefit Costs [Member]
Regulatory assets:
Regulatory assets	296.4	240.6
Postretirement Benefit Costs [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	296.5	238.3
Deferred Bond Refinancing Costs [Member]
Regulatory assets:
Regulatory assets	7.1	6.5
Deferred Bond Refinancing Costs [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	5.9	6.5
Debt Basis Adjustment [Member]
Regulatory assets:
Regulatory assets	14.9	17.5
Competitive Rate Adjustment [Member]
Regulatory assets:
Regulatory assets	2.5	2.6
Competitive Rate Adjustment [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	2.5	2.6
Other [Member]
Regulatory assets:
Regulatory assets	14.8	12.1
Other [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	10.1	10.7
Regulatory Tax Liability [Member]
Regulatory liabilities:
Regulatory liabilities	7.2	7.9
Regulatory Tax Liability [Member] | Tampa Electric Company [Member]
Regulatory liabilities:
Regulatory liabilities	5.3	5.7
Cost-Recovery Clauses [Member]
Regulatory liabilities:
Regulatory liabilities	111.5	55.9
Cost-Recovery Clauses [Member] | Tampa Electric Company [Member]
Regulatory liabilities:
Regulatory liabilities	107.8	54.2
Transmission and Delivery Storm Reserve [Member]
Regulatory liabilities:
Regulatory liabilities	56.1	56.1
Transmission and Delivery Storm Reserve [Member] | Tampa Electric Company [Member]
Regulatory liabilities:
Regulatory liabilities	56.1	56.1
Accumulated Reserve - Cost of Removal [Member]
Regulatory liabilities:
Regulatory liabilities	670.6	679.9
Accumulated Reserve - Cost of Removal [Member] | Tampa Electric Company [Member]
Regulatory liabilities:
Regulatory liabilities	554.8	570.0
Bill Reduction Credit [Member]
Regulatory liabilities:
Regulatory liabilities	8.0	0.3
Other [Member]
Regulatory liabilities:
Regulatory liabilities	13.6	0.5
Other [Member] | Tampa Electric Company [Member]
Regulatory liabilities:
Regulatory liabilities	$ 13.4	$ 0.7