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Debt - Narrative (Details)	3 Months Ended				9 Months Ended
Debt - Narrative (Details)	Jun. 30, 2019 CAD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 CAD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2019 CAD ($)	Sep. 30, 2019 USD ($)	Sep. 27, 2019 USD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
Commercial Paper					$ 603,000,000	$ 455,000,000		$ 0
Revolving Credit Facility
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity						1,300,000,000			$ 1,000,000,000.0
Current borrowing capacity						1,300,000,000
U.S $1.0 Billion Tranche
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity							$ 1,000,000,000.0
U.S. $3 Million Tranche
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity							$ 300,000,000
Bilateral Letter of Credit Facilities
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 300,000,000			$ 600,000,000
Commercial Paper
Debt Instrument [Line Items]
Current borrowing capacity						$ 1,000,000,000.0
Weighted average interest rate, Commercial paper					2.38%
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%
Debt instrument, Term	10 years	10 years