SHARE CAPITAL - COMMON SHARES (Details) shares in Millions, CAD in Millions	12 Months Ended
	Dec. 31, 2017 CAD shares	Dec. 31, 2016 CAD shares	Dec. 31, 2015 CAD shares	Dec. 31, 2017 $ / shares
Share Capital
Common shares par value (in dollars per share) | $ / shares				$ 0
Common Shares
Balance at beginning of period (in shares) | shares	943
Balance at end of period (in shares) | shares	1,695	943
Gross proceeds | CAD	CAD 1,549	CAD 2,260	CAD 57
Issuance costs | CAD	CAD 0	CAD 59	CAD 0
Common shares
Common Shares
Balance at beginning of period (in shares) | shares	943	868	852
Balance at beginning of period, Amount | CAD	CAD 10,492	CAD 7,391	CAD 6,669
Common shares issued (in shares) | shares	33	56	0
Common shares issued, Amount | CAD	CAD 1,500	CAD 2,241	CAD 0
Common shares issued in Merger Transaction (in shares) | shares	691	0	0
Common shares issued in Merger Transaction, Amount | CAD	CAD 37,429	CAD 0	CAD 0
Dividend Reinvestment and Share Purchase Plan (in shares) | shares	25	16	12
Dividend Reinvestment and Share Purchase Plan | CAD	CAD 1,226	CAD 795	CAD 646
Shares issued on exercise of stock options (in shares) | shares	3	3	4
Shares issued on exercise of stock options, Amount | CAD	CAD 90	CAD 65	CAD 76
Balance at end of period (in shares) | shares	1,695	943	868
Balance at end of period, Amount | CAD	CAD 50,737	CAD 10,492	CAD 7,391