Income Taxes - Narrative (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount	$ (88)
Effect of tax rate decrease	95	$ 21	$ 541
Corporate income tax rate before Tax Cuts and Jobs Act			35.00%
Corporate income tax rate after Tax Cuts and Jobs Act		21.00%
Income tax operating losses carried forward	4	$ 8
Minimum tax credits or investment tax credits carried forward	0
Capital losses carried forward	2	3
Uncertain tax position liability	24
Investment tax credits carried forward		0
Net amount of accrued interest and penalties	1	0	$ 1
Total amount of accrued interest and penalties associated with unrecognized tax benefit	$ 10	$ 11	$ 11