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Fair Value Measurements and Financial Instruments - Derivative Instruments (Details) CAD in Millions, $ in Millions | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2016 CAD | Dec. 31, 2016 USD ($) | Dec. 31, 2016 CAD | Oct. 14, 2016 USD ($) | Dec. 31, 2015 CAD | Dec. 31, 2014 CAD
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Unrealized losses recognized in regulatory assets |  |  | CAD 2,933 |  | CAD 2,532 |
Unrealized gains recognized in regulatory liabilities |  |  | 2,183 |  | 1,638 |
Cash flow hedges |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Unrealized gain on the derivatives |  |  | 8 |  | 3 | CAD 1
Aitken Creek |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Unrealized losses on gas swap contracts | CAD 9 |  |  |  |  |
Unrealized losses on gas swap contracts, after tax | CAD 6 |  |  |  |  |
UNS Energy |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Realized Gains Shared With Customers, Percent | 10.00% |  |  |  |  |
ITC | Cash flow hedges |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Unrealized gain on the derivatives |  | $ 8 | 11 |  |  |
Derivative instruments |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Unrealized losses recognized in regulatory assets |  |  | 19 |  | 74 |
Unrealized gains recognized in regulatory liabilities |  |  | CAD 12 |  | CAD 3 |
Unsecured | ITC |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Face value | $ |  | 385 |  |  |  |
Unsecured | ITC |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Face value | $ |  |  |  | $ 2,000 |  |
Interest rate swaps | Cash flow hedges |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Loss expected to be reclassified to earnings | CAD 5 |  |  |  |  |
Interest rate swaps | Designated as hedging instrument | Cash flow hedges | ITC |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Notional amount | $ |  | $ 100 |  |  |  |

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Amount before tax of increase (decrease) in accumulated equity from transactions and other events and circumstances from non-owner sources, including portion attributable to noncontrolling interest. Excludes net income (loss), and accumulated changes in equity from transactions resulting from investments by owners (distributions to owners). + References No definition available. + Details
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- Definition The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994 + Details
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