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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	$ (2,009)	$ (1,673)	$ (2,043)	$ (1,741)
Other comprehensive income (loss) before reclassifications	1	(2)	(1)	19
Amounts reclassified from accumulated other comprehensive loss	16	21	52	68
Net current-period other comprehensive income (loss)	17	19	51	87
AOC Income (Loss), Closing balance	(1,992)	(1,654)	(1,992)	(1,654)
Foreign currency net of hedging activities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	112	110	113	109
Other comprehensive income (loss) before reclassifications	1	(1)	0	0
Amounts reclassified from accumulated other comprehensive loss	0	0	0	0
Net current-period other comprehensive income (loss)	1	(1)	0	0
AOC Income (Loss), Closing balance	113	109	113	109
Derivatives and other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOC Income (Loss), Opening balance	(58)	(64)	(62)	(89)
Other comprehensive income (loss) before reclassifications	0	(2)	0	19
Amounts reclassified from accumulated other comprehensive loss	1	2	5	6
Net current-period other comprehensive income (loss)	1	0	5	25
AOC Income (Loss), Closing balance	(57)	(64)	(57)	(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	(2,063)	(1,719)	(2,094)	(1,761)
Other comprehensive income (loss) before reclassifications	0	1	(1)	0
Amounts reclassified from accumulated other comprehensive loss	15	19	47	62
Net current-period other comprehensive income (loss)	15	20	46	62
AOC Income (Loss), Closing balance	$ (2,048)	$ (1,699)	$ (2,048)	$ (1,699)