Debt - Credit Facility (Narrative) (Details) $ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019 CAD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2018 USD ($)
Commercial Paper
Debt Instrument [Line Items]
Credit facility, Total available amount		$ 1,000
Commercial paper	$ 516	397
Commercial paper, Weighted-average interest rate	2.03%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility, Total available amount		1,300		$ 1,000
Remaining borrowing capacity		1,300		$ 1,000
Five-Year Portion [Member]
Debt Instrument [Line Items]
Credit facility, Total available amount		1,000
One-Year Plus One-Year Term-Out Portion [Member]
Debt Instrument [Line Items]
Credit facility, Total available amount		$ 300
Letters of Credit [Member]
Debt Instrument [Line Items]
Letters of Credit, Drawn	$ 80		$ 60
Maximum capacity under credit facility	$ 300		$ 600