Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Amounts in Pension and Post-retirement Defined Benefit Plans Reclassified from AOCL (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income tax expense (recovery)	$ 124	$ 122	$ 263	$ 244
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	1	0	1	(1)
Recognition of net actuarial loss	20	29	41	59
Total before income tax	21	29	42	58
Income tax expense (recovery)	(5)	(8)	(10)	(15)
Total net of income tax	$ 16	$ 21	$ 32	$ 43