CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 664	$ 618	$ 724	$ 434	$ 545	$ 622	$ 436	$ 348	$ 2,440	$ 1,951	$ 2,405
Net gain (loss) in foreign currency translation adjustments, net of hedging activities									37	(60)	24
Change in derivatives designated as cash flow hedges									10	38	19
Change in pension and post-retirement defined benefit plans									(661)	(449)	80
Other comprehensive income (loss) before income taxes									(614)	(471)	123
Income tax recovery (expense) on above items									135	169	(65)
Other comprehensive (loss) income									(479)	(302)	58
Comprehensive income									$ 1,961	$ 1,649	$ 2,463