Debt - Credit Facility (Narrative) (Details) $ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2020 CAD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 CAD ($)
Commercial Paper
Debt Instrument [Line Items]
Commercial paper		$ 820	$ 644	$ 516
Weighted-average interest rate, Commercial paper		0.27%	0.27%	2.03%
Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility, Total available amount			$ 1,300
U.S. $1.0 Billion Tranche
Debt Instrument [Line Items]
Credit facility, Total available amount			1,000
U.S. $300 Million Tranche
Debt Instrument [Line Items]
Credit facility, Total available amount			$ 300
Credit facility, Drawn	$ 100
Letters of Credit
Debt Instrument [Line Items]
Letters of Credit, Drawn		$ 59		$ 80
Maximum capacity under credit facility		$ 300		$ 300