Credit Facilities, Long-Term Debt, and Finance Lease Obligations - Non-Recourse Debt (Details) - CAD CAD in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2017
Disclosure of detailed information about borrowings [line items]
Borrowings, adjustment to interest rate basis	3.95%
Interest rate	6.65%	6.65%
Notional amount	CAD 4,311.0	CAD 3,664.0
Borrowings	CAD 4,361.0	CAD 3,707.0
5.69% Non-Recourse Debenture due 2016 \| Non-recourse debt
Disclosure of detailed information about borrowings [line items]
Interest rate	5.69%
Notional amount	CAD 27.0
Non-Recourse Bonds \| Non-recourse debt
Disclosure of detailed information about borrowings [line items]
Redemption of noncurrent borrowings	10.0
Loss on early redemption of bonds	CAD 1.0
Non-Recourse Variable Rate Debt \| Minimum
Disclosure of detailed information about borrowings [line items]
Interest rate	2.95%
Non-Recourse Variable Rate Debt \| Maximum
Disclosure of detailed information about borrowings [line items]
Interest rate	5.36%
New Richmond \| 3.963% Non-Recourse Bond due 2032 \| Non-recourse debt
Disclosure of detailed information about borrowings [line items]
Interest rate	3.963%
Notional amount	CAD 159.0
Principal payment	4.0
Melancthon-Wolfe Wind \| 3.834% Non-Recourse Bond due 2028 \| Non-recourse debt
Disclosure of detailed information about borrowings [line items]
Principal payment	35.0
TAPC Holdings \| Variable Rate Bond due 2030 \| Non-recourse debt
Disclosure of detailed information about borrowings [line items]
Notional amount	CAD 202.5
TAPC Holdings \| Variable Rate Bond due 2030 \| Non-recourse debt \| Canadian Dollar Offered Rate
Disclosure of detailed information about borrowings [line items]
Borrowings, adjustment to interest rate basis	3.95%

The amount of outstanding funds that the entity is obligated to repay.

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The adjustment to the basis (reference rate) used for calculation of the interest rate on borrowings. [Refer: Borrowings]

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The cash outflow for repayments of non-current borrowings. [Refer: Borrowings]

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