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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before income tax	$ 821	$ 680	$ 1,849	$ 1,877
Income tax recovery	(199)	(170)	(443)	(456)
Net of income tax	622	510	1,406	1,421
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	(1)	(1)	(2)	(3)
Recognition of net actuarial loss	29	39	88	116
Total before income tax	28	38	86	113
Income tax recovery	(9)	(10)	(24)	(30)
Net of income tax	$ 19	$ 28	$ 62	$ 83