Changes in Accumulated Other Comprehensive Loss ("AOCL") by Component - Changes in Accumulated Other Comprehensive Loss (AOCL) by Component (Details) - CAD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,027)	$ (1,705)	$ (2,043)	$ (1,741)
Other comprehensive income (loss) before reclassifications	0	9	(2)	21
Amounts reclassified from accumulated other comprehensive loss	18	23	36	47
Net current-period other comprehensive income (loss)	18	32	34	68
AOC Income (Loss), Closing balance	(2,009)	(1,673)	(2,009)	(1,673)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	113	109	113	109
Other comprehensive income (loss) before reclassifications	(1)	1	(1)	1
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive income (loss)	(1)	1	(1)	1
AOC Income (Loss), Closing balance	112	110	112	110
Derivatives and other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(61)	(74)	(62)	(89)
Other comprehensive income (loss) before reclassifications	1	8	0	21
Amounts reclassified from accumulated other comprehensive loss	2	2	4	4
Net current-period other comprehensive income (loss)	3	10	4	25
AOC Income (Loss), Closing balance	(58)	(64)	(58)	(64)
Pension and post-retirement defined benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(2,079)	(1,740)	(2,094)	(1,761)
Other comprehensive income (loss) before reclassifications	0	0	(1)	(1)
Amounts reclassified from accumulated other comprehensive loss	16	21	32	43
Net current-period other comprehensive income (loss)	16	21	31	42
AOC Income (Loss), Closing balance	$ (2,063)	$ (1,719)	$ (2,063)	$ (1,719)