Regulatory - Schedule of Regulatory Assets and Regulatory Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Regulatory assets:
Regulatory assets	$ 373.9	$ 402.1
Less: Current portion	43.1	53.6
Long-term regulatory assets	330.8	348.5
Regulatory liabilities:
Regulatory liabilities	785.3	786.0
Less: Current portion	69.1	57.0
Long-term regulatory liabilities	716.2	729.0
Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	345.1	371.7
Less: Current portion	39.7	52.1
Long-term regulatory assets	305.4	319.6
Regulatory liabilities:
Regulatory liabilities	671.4	678.1
Less: Current portion	65.7	54.7
Long-term regulatory liabilities	605.7	623.4
Regulatory Tax Asset [Member]
Regulatory assets:
Regulatory assets	73.1	69.2
Regulatory Tax Asset [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	73.0	69.2
Cost-Recovery Clauses - Deferred Balances [Member]
Regulatory assets:
Regulatory assets	2.8	1.9
Cost-Recovery Clauses - Deferred Balances [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	0.1	0.9
Cost-Recovery Clauses - Offsets to Derivative Liabilities [Member]
Regulatory assets:
Regulatory assets	28.1	43.2
Cost-Recovery Clauses - Offsets to Derivative Liabilities [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	27.4	42.7
Environmental Remediation [Member]
Regulatory assets:
Regulatory assets	52.6	53.1
Environmental Remediation [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	52.6	53.1
Postretirement Benefit Costs [Member]
Regulatory assets:
Regulatory assets	177.4	194.0
Postretirement Benefit Costs [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	172.0	187.8
Deferred Bond Refinancing Costs [Member]
Regulatory assets:
Regulatory assets	6.6	7.2
Deferred Bond Refinancing Costs [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	6.6	7.2
Debt Basis Adjustment [Member]
Regulatory assets:
Regulatory assets	18.3	20.9
Competitive Rate Adjustment [Member]
Regulatory assets:
Regulatory assets	2.4	2.8
Competitive Rate Adjustment [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	2.4	2.8
Other [Member]
Regulatory assets:
Regulatory assets	12.6	9.8
Other [Member] | Tampa Electric Company [Member]
Regulatory assets:
Regulatory assets	11.0	8.0
Regulatory Tax Liability [Member]
Regulatory liabilities:
Regulatory liabilities	6.2	6.9
Regulatory Tax Liability [Member] | Tampa Electric Company [Member]
Regulatory liabilities:
Regulatory liabilities	4.3	5.1
Cost-Recovery Clauses [Member]
Regulatory liabilities:
Regulatory liabilities	40.2	25.9
Cost-Recovery Clauses [Member] | Tampa Electric Company [Member]
Regulatory liabilities:
Regulatory liabilities	36.6	23.5
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	682.1	695.2
Accumulated Reserve - Cost of Removal [Member] | Tampa Electric Company [Member]
Regulatory liabilities:
Regulatory liabilities	573.7	591.5
Other [Member]
Regulatory liabilities:
Regulatory liabilities	0.7	1.9
Other [Member] | Tampa Electric Company [Member]
Regulatory liabilities:
Regulatory liabilities	$ 0.7	$ 1.9