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Fair Value of Financial Instruments and Risk Management - Foreign Exchange Hedge (Narrative) (Details) - Foreign net investments - USD ($) $ in Billions	Dec. 31, 2024	Dec. 31, 2023
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Unhedged foreign net investments	$ 12.6	$ 11.5
Designated as hedginginstrument
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Long-term debt designated as an effective hedge	$ 2.2	$ 2.6

X

- Definition

Net Investment In Foreign Operation, Amount Not Hedged

+ References

No definition available.

+ Details

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Namespace Prefix:	fts_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of hedged item in hedging relationship. Excludes hedged asset or liability.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

X

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosuresLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: