Debt - Credit Facility - Narrative (Details) $ in Millions	Dec. 31, 2021 CAD ($)	Dec. 31, 2021 USD ($)	Sep. 24, 2021 CAD ($)	Sep. 24, 2021 USD ($)	Dec. 31, 2020 CAD ($)
Commercial Paper
Debt Instrument [Line Items]
Commercial paper	$ 336	$ 265,000,000			$ 820
Weighted-average interest rate, commercial paper	0.32%	0.32%			0.27%
Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility, available amount		$ 1,300,000,000
U.S. $1.0 Billion Tranche
Debt Instrument [Line Items]
Credit facility, available amount		1,000,000,000		$ 1,000,000,000
U.S. $300 Million Tranche
Debt Instrument [Line Items]
Credit facility, available amount			$ 300
Term Credit Facility
Debt Instrument [Line Items]
Debt face amount		500,000,000
Amount outstanding	$ 634	$ 500,000,000
Weighted-average interest rate	1.38%	1.38%
Letters of Credit
Debt Instrument [Line Items]
Letters of credit drawn	$ 58				$ 59
Maximum capacity under credit facility	$ 300