CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - CAD CAD in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	CAD 984	CAD 510	CAD 480	CAD 431	CAD 384	CAD 347	CAD 328	CAD 540	CAD 2,405	CAD 1,599	CAD 1,352
Net gain (loss) in foreign currency translation adjustments, net of hedging activities									24	18	(86)
Change in derivatives designated as cash flow hedges									19	(2)	(69)
Change in pension and post-retirement defined benefit plans									80	(434)	1,059
Other comprehensive income (loss) before income taxes									123	(418)	904
Income tax (expense) recovery on above items									(65)	96	(162)
Other comprehensive income (loss)									58	(322)	742
Comprehensive income									CAD 2,463	CAD 1,277	CAD 2,094