Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2015 USD ($) Segment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Equity method investment information	In certain circumstances this can result in the company consolidating entities in which it has less than a 50% equity investment and deconsolidating entities in which it has a majority equity interest
Equity method investment, Ownership percentage	50.00%
Percentage of original cost of depreciable property	3.70%	3.60%	3.70%
Depreciation expense	$ 339.1	$ 307.5	$ 285.6
Allowance for funds used during construction	26.1	15.8	9.9
Unbilled revenues	81.1	86.6
Purchased power	78.9	71.4	64.7
Franchise fees and gross receipts taxes	$ 116.9	$ 113.9	$ 108.5
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Discount rates used in estimating other self-insurance liabilities	2.92%	2.71%
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Discount rates used in estimating other self-insurance liabilities	4.00%	4.00%
Computer Software [Member] \| Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization period	3 years
Computer Software [Member] \| Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization period	15 years
Tampa Electric Company [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of business segments \| Segment	2
Equity method investment information	In certain circumstances this can result in TEC consolidating entities in which it has less than a 50% equity investment and deconsolidating entities in which it has a majority equity interest
Equity method investment, Ownership percentage	50.00%
Percentage of original cost of depreciable property	3.70%	3.70%	3.70%
Depreciation expense	$ 306.0	$ 295.8	$ 284.2
Allowance for funds used during construction rate	6.46%	6.46%	8.16%
Allowance for funds used during construction	$ 25.5	$ 15.6	$ 9.9
Unbilled revenues	53.7	49.3
Purchased power	78.9	71.4	64.7
Franchise fees and gross receipts taxes	$ 116.9	$ 113.9	108.5
Tampa Electric Company [Member] \| Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Discount rates used in estimating other self-insurance liabilities	2.92%	2.71%
Tampa Electric Company [Member] \| Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Discount rates used in estimating other self-insurance liabilities	4.00%	4.00%
Tampa Electric Company [Member] \| Computer Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Amortization period	15 years	15 years
NMGC [Member]
Summary Of Significant Accounting Policies [Line Items]
Allowance for funds used during construction rate	4.41%	4.92%
Rights of way	$ 41.0	$ 41.0
Rights of way, accumulated amortization	9.0	8.0
TECO Energy Source, Inc [Member]
Summary Of Significant Accounting Policies [Line Items]
Costs netted against revenues	$ 3.1	$ 4.3	$ 23.1

Franchise fees and gross receipts taxes.

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Summary of significant accounting policies.

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Cost of electricity purchased as well as fuel used to generate electricity used in operations and sold during the reporting period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
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The aggregate cost of goods produced and sold and services rendered during the reporting period.

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The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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This serves as a place to record data that is not required by accounting literature but is useful for readers of the financial statements as it relates to the details of an equity method investment in common stock. Such information may include the amount of change for a change in ownership (equity) percentage in a given period and the amount and type of the consideration given or received in relation to the change in ownership.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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The component of the allowance for funds used during construction during the period based on an assumed rate of return on equity funds used in financing the construction of regulated assets.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Effective rate, including composite rate, of cost of financing additions to qualifying regulated assets, commonly called allowance for funds used during construction.

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Useful life of assets common to business units owned by public utility, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Composite depreciation rate for public utility plants in service.

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Amount received for services rendered and products shipped, but not yet billed, for non-contractual agreements due within one year or the normal operating cycle, if longer.

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The average discount rate applied to insurance contracts to determine future policy benefits to be paid to or on behalf of policyholders.

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