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Commitments - Fiscal Year Maturity (Details) - Dec. 31, 2016 CAD in Millions, $ in Millions | USD ($) | CAD
--- | --- | ---
Operating lease obligations: |  |
Total |  | CAD 175
Due within 1 year |  | 13
Due in year 2 |  | 13
Due in year 3 |  | 11
Due in year 4 |  | 8
Due in year 5 |  | 7
Due after 5 years |  | 123
Promissory note: |  |
Due within 1 year |  | 251
Due in year 2 |  | 931
Due in year 3 |  | 679
Due in year 4 |  | 725
Due in year 5 |  | 1,756
Due after 5 years |  | 16,877
Total |  | 22,135
Due within 1 year |  | 2,027
Due in year 2 |  | 1,666
Due in year 3 |  | 1,451
Due in year 4 |  | 1,398
Due in year 5 |  | 1,318
Due after 5 years |  | 14,275
Power |  |
Purchase obligations: |  |
Total |  | 2,295
Due within 1 year |  | 290
Due in year 2 |  | 200
Due in year 3 |  | 119
Due in year 4 |  | 107
Due in year 5 |  | 107
Due after 5 years |  | 1,472
Renewable power |  |
Purchase obligations: |  |
Total |  | 1,625
Due within 1 year |  | 100
Due in year 2 |  | 99
Due in year 3 |  | 99
Due in year 4 |  | 98
Due in year 5 |  | 97
Due after 5 years |  | 1,132
Gas |  |
Purchase obligations: |  |
Total |  | 1,329
Due within 1 year |  | 411
Due in year 2 |  | 290
Due in year 3 |  | 177
Due in year 4 |  | 141
Due in year 5 |  | 110
Due after 5 years |  | 200
Renewable energy credit purchase agreement |  |
Purchase obligations: |  |
Total |  | 154
Due within 1 year |  | 20
Due in year 2 |  | 15
Due in year 3 |  | 12
Due in year 4 |  | 12
Due in year 5 |  | 12
Due after 5 years |  | 83
Interest obligations on long-term debt |  |
Other commitments: |  |
Total |  | 14,586
Due within 1 year |  | 892
Due in year 2 |  | 854
Due in year 3 |  | 837
Due in year 4 |  | 817
Due in year 5 |  | 793
Due after 5 years |  | 10,393
Purchase of Springerville Common Facilities |  |
Other commitments: |  |
Total |  | 91
Due within 1 year |  | 0
Due in year 2 |  | 0
Due in year 3 |  | 0
Due in year 4 |  | 0
Due in year 5 |  | 91
Due after 5 years |  | 0
Joint-use asset and shared service agreements |  |
Other commitments: |  |
Total |  | 53
Due within 1 year |  | 3
Due in year 2 |  | 3
Due in year 3 |  | 3
Due in year 4 |  | 3
Due in year 5 |  | 3
Due after 5 years |  | 38
Other |  |
Other commitments: |  |
Total |  | 156
Due within 1 year |  | 93
Due in year 2 |  | 18
Due in year 3 |  | 19
Due in year 4 |  | 0
Due in year 5 |  | 0
Due after 5 years |  | 26
UNS Energy | Long-term contracts - UNS Energy |  |
Purchase obligations: |  |
Total |  | 1,146
Due within 1 year |  | 192
Due in year 2 |  | 161
Due in year 3 |  | 161
Due in year 4 |  | 127
Due in year 5 |  | 85
Due after 5 years |  | 420
UNS Energy | Renewable energy credit purchase agreement |  |
Purchase obligations: |  |
Total | $ | $ 107 |
ITC | ITC easement agreement |  |
Other commitments: |  |
Total |  | 453
Due within 1 year |  | 13
Due in year 2 |  | 13
Due in year 3 |  | 13
Due in year 4 |  | 13
Due in year 5 |  | 13
Due after 5 years |  | 388
Waneta Partnership | Waneta Partnership promissory note |  |
Promissory note: |  |
Total |  | 72
Due within 1 year |  | 0
Due in year 2 |  | 0
Due in year 3 |  | 0
Due in year 4 |  | 72
Due in year 5 |  | 0
Due after 5 years |  | CAD 0

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

X
- Definition Minimum amount of purchase arrangement maturing in the next fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Minimum amount of purchase arrangement maturing in the second fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Minimum amount of purchase arrangement maturing in the third fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
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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: |

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- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=fts_RenewablePowerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=fts_GasTransportationandStorageServicesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=fts_EnvironmentalAttributionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_OtherCommitmentsAxis=fts_InterestObligationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=fts_CoalandGasPurchaseandDeliveryandTransmissionServicesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | dei_LegalEntityAxis=fts_ITCHoldingsCorp.Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_OtherCommitmentsAxis=fts_EasementAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=us-gaap_NotesPayableOtherPayablesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |