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Debt - Narrative (Details)	3 Months Ended				6 Months Ended
Debt - Narrative (Details)	Jun. 30, 2019 CAD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 CAD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 CAD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 CAD ($)
Debt Instrument [Line Items]
Commercial Paper	$ 242,000,000				$ 242,000,000	$ 185,000,000	$ 0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Current borrowing capacity						1,000,000,000.0
Commercial Paper [Member]
Debt Instrument [Line Items]
Current borrowing capacity						$ 1,000,000,000.0
Weighted average interest rate, Commercial paper					2.64%
CAD $400 million 3.150% 10-year Notes
Debt Instrument [Line Items]
Debt instrument, Interest rate			3.15%	4.00%
Debt instrument, Term			10 years	10 years
Debt instrument, Face amount			$ 400,000,000	$ 500,000,000
Debt instrument, Maturity date			Mar. 13, 2029
Proceeds from issuance of debt			$ 397,000,000
U.S. $350 million 7.250% 10-year Notes
Debt Instrument [Line Items]
Retirement of long-term debt, Amount	$ 471,000,000	$ 350,000,000
Debt instrument, Interest rate	7.25%				7.25%	7.25%
Debt instrument, Term	10 years	10 years