CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - CAD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 545	$ 622	$ 436	$ 348	$ 984	$ 510	$ 480	$ 431	$ 1,951	$ 2,405	$ 1,599
Net (loss) gain in foreign currency translation adjustments, net of hedging activities									(60)	24	18
Change in derivatives designated as cash flow hedges									38	19	(2)
Change in pension and post-retirement defined benefit plans									(449)	80	(434)
Other comprehensive (loss) income before income taxes									(471)	123	(418)
Income tax recovery (expense) on above items									169	(65)	96
Other comprehensive (loss) income									(302)	58	(322)
Comprehensive income									$ 1,649	$ 2,463	$ 1,277