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Acquision (Related Expenses and Pro Forma Data) (Details) - TECO Energy Inc [Member] - CAD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Business Combination, Separately Recognized Transactions [Line Items] |  |
Business Combination, Acquisition Related Costs | $ 0 |
Business Combination, Acquisition Related Costs, Net of Tax |  | $ 166
Business Acquisition, Pro Forma Information [Abstract] |  |
Pro forma operating revenues |  | 6,034
Prop forma net income attributable to common shareholders |  | 386
Business Acquisitions Pro Forma Increase To Net Income Loss |  | 53
Sales [Member] |  |
Business Combination, Separately Recognized Transactions [Line Items] |  |
Business Combination, Acquisition Related Costs | 0 | (10)
Operating Expense [Member] |  |
Business Combination, Separately Recognized Transactions [Line Items] |  |
Business Combination, Acquisition Related Costs | 0 | 89
Interest Expense [Member] |  |
Business Combination, Separately Recognized Transactions [Line Items] |  |
Business Combination, Acquisition Related Costs | 0 | 148
Other Operating Income (Expense) [Member] |  |
Business Combination, Separately Recognized Transactions [Line Items] |  |
Business Combination, Acquisition Related Costs | 0 | (3)
Income Tax Expense [Member] |  |
Business Combination, Separately Recognized Transactions [Line Items] |  |
Business Combination, Acquisition Related Costs | $ 0 | $ (84)

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