CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - CAD CAD in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	CAD 384	CAD 347	CAD 328	CAD 540	CAD 319	CAD 323	CAD 390	CAD 320	CAD 1,599	CAD 1,352	CAD 1,476
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent [Abstract]
Net gain (loss) in foreign currency translation adjustments, net of hedging activities									18	(86)	(32)
Change in derivatives designated as cash flow hedges									(2)	(69)	(49)
Change in pension and post-retirement defined benefit plans									(434)	1,059	(941)
Other comprehensive income before income taxes									(418)	904	(1,022)
Income tax recovery (expense) on above items									96	(162)	306
Other comprehensive income									(322)	742	(716)
Comprehensive income									CAD 1,277	CAD 2,094	CAD 760