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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income tax recovery	$ (122)	$ (138)	$ (244)	$ (286)
Net of income tax	436	480	784	911
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service costs	0	(1)	(1)	(2)
Recognition of net actuarial loss	29	38	59	77
Total before income tax	29	37	58	75
Income tax recovery	(8)	(10)	(15)	(20)
Net of income tax	$ 21	$ 27	$ 43	$ 55