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Regulatory Assets and Liabilities (Storm reserve & Solar investment) (Details) $ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) MW	Dec. 31, 2016 USD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2017 USD ($)	Sep. 10, 2017 USD ($)	Dec. 31, 2016 CAD ($)
Regulatory Assets [Line Items]
Regulatory Assets			$ 1,376			$ 1,322
Tampa Electric Division [Member] | Tropical Storm Colin, Hurricane Hermine and Hurricane Matthew Restoration [Member] | OM&G [Member] | Other Regulatory Assets (Liabilities) [Member] | LossFromCatastrophesMember
Regulatory Assets [Line Items]
Storm cost		$ 10
Tampa Electric Division [Member] | Hurricane Irma Restoration [Member] | OM&G [Member] | Capital Expenditures [Member] | LossFromCatastrophesMember
Regulatory Assets [Line Items]
Storm Reserve					$ 8
Tampa Electric Division [Member] | Hurricane Irma Restoration [Member] | OM&G [Member] | Other Regulatory Assets (Liabilities) [Member] | LossFromCatastrophesMember
Regulatory Assets [Line Items]
Storm cost	$ 4
Storm Reserve					$ 93
Tampa Electric Division [Member] | SoBRA [Member] | Capital Expenditures [Member]
Regulatory Assets [Line Items]
Estimated Investement in projects				$ 850
Capacity Of Utility Project | MW	600
Estimated revenue	$ 26
Estimated revenue in MW | MW	145

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- References

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The estimated amount of investment in public utility projects over a period of time.

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The estimated revenue from a public utility project.

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The estimated amount of power generation from a public utility project.

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- Definition

Carrying amount as of the balance sheet date of capitalized storm costs of a regulated entity that are expected to be recoverable through rate adjustments after one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77907324&loc=d3e43603-110378

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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- Definition

A noncash adjustment to the provision established for storm damage by an oil, gas, or utility company.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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