SHARE CAPITAL - PREFERRED SHARES (Details) CAD / shares in Units, CAD in Millions											12 Months Ended
	Mar. 01, 2018 CAD / shares	Feb. 28, 2018 CAD / shares	Dec. 01, 2017 CAD / shares	Sep. 01, 2017 CAD / shares	Sep. 01, 2017 $ / shares	Aug. 31, 2017 $ / shares	Jun. 01, 2017 CAD / shares shares	Jun. 01, 2017 $ / shares shares	May 31, 2017 CAD / shares	May 31, 2017 $ / shares	Dec. 31, 2017 CAD CAD / shares shares	Dec. 31, 2017 CAD CAD / shares $ / shares shares	Dec. 31, 2016 CAD shares	Dec. 31, 2015 CAD shares	Feb. 14, 2018 shares	Dec. 31, 2017 $ / shares
Preference Shares
Issuance costs | CAD											CAD 0		CAD (59)	CAD 0
Convertible Preferred Stock, Shares Issued upon Conversion | shares							1,730,188	1,730,188
Preference Shares, Series B
Preference Shares
Yearly dividend per share (in dollars per share)							CAD 0.21340		CAD 0.25000
Preference Shares, Series C
Preference Shares
Yearly dividend per share (in dollars per share)			CAD 0.20342	CAD 0.19571			CAD 0.18600
Preference Shares, Series J
Preference Shares
Yearly dividend per share (in dollars per share) | $ / shares								$ 0.30540		$ 0.25000
Preference Shares, Series L
Preference Shares
Yearly dividend per share (in dollars per share) | $ / shares					$ 0.30993	$ 0.25000
Preference shares
Preference Shares
Preferred stock, value, outstanding | CAD											7,747	CAD 7,747	7,255	6,515
Issuance costs | CAD											CAD (155)		CAD (147)	CAD (137)
Recurring anniversary period following the redemption option date, at which the entity may redeem preferred shares											5 years
Stock split conversion ratio											1
Quarterly floating rate cumulative dividends per share calculation, base multiplier (in dollars per share) | (per share)											CAD 25	CAD 25
Quarterly floating rate cumulative dividends per share calculation, period of calendar year											365 days
Quarterly floating rate cumulative dividends per share calculation, maturity period of Government of Canada treasury bill											90 days
Quarterly floating rate cumulative dividends per share calculation, maturity period of Government of US treasury bill											3 months
Preference shares | Preference Shares, Series A
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											5,000,000	5,000,000	5,000,000	5,000,000
Preferred stock, value, outstanding | CAD											CAD 125	CAD 125	CAD 125	CAD 125
Initial Yield (as a percent)											5.50%
Yearly dividend per share (in dollars per share)											CAD 1.37500
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series B
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											18,000,000	18,000,000	20,000,000	20,000,000
Preferred stock, value, outstanding | CAD											CAD 457	CAD 457	CAD 500	CAD 500
Initial Yield (as a percent)											3.415%
Yearly dividend per share (in dollars per share)											CAD 0.85360
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series C
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											2,000,000	2,000,000	0	0
Preferred stock, value, outstanding | CAD											CAD 43	CAD 43	CAD 0	CAD 0
Initial Yield (as a percent)											2.40%
Yearly dividend per share (in dollars per share)											CAD 0
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.40%
Preference shares | Preference Shares, Series D
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											18,000,000	18,000,000	18,000,000	18,000,000
Preferred stock, value, outstanding | CAD											CAD 450	CAD 450	CAD 450	CAD 450
Initial Yield (as a percent)											4.00%
Yearly dividend per share (in dollars per share)											CAD 1.00000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series F
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											20,000,000	20,000,000	20,000,000	20,000,000
Preferred stock, value, outstanding | CAD											CAD 500	CAD 500	CAD 500	CAD 500
Initial Yield (as a percent)											4.00%
Yearly dividend per share (in dollars per share)											CAD 1.00000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series H
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											14,000,000	14,000,000	14,000,000	14,000,000
Preferred stock, value, outstanding | CAD											CAD 350	CAD 350	CAD 350	CAD 350
Initial Yield (as a percent)											4.00%
Yearly dividend per share (in dollars per share)											CAD 1.00000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series J
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											8,000,000	8,000,000	8,000,000	8,000,000
Preferred stock, value, outstanding | CAD											CAD 199	CAD 199	CAD 199	CAD 199
Initial Yield (as a percent)											4.887%
Yearly dividend per share (in dollars per share) | $ / shares												CAD 1.22160
Per Share Base Redemption Value (in dollars per shares) | $ / shares																$ 25
Preference shares | Preference Shares, Series L
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											16,000,000	16,000,000	16,000,000	16,000,000
Preferred stock, value, outstanding | CAD											CAD 411	CAD 411	CAD 411	CAD 411
Initial Yield (as a percent)											4.959%
Yearly dividend per share (in dollars per share) | $ / shares												CAD 1.23972
Per Share Base Redemption Value (in dollars per shares) | $ / shares																25
Preference shares | Preference Shares, Series N
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											18,000,000	18,000,000	18,000,000	18,000,000
Preferred stock, value, outstanding | CAD											CAD 450	CAD 450	CAD 450	CAD 450
Initial Yield (as a percent)											4.00%
Yearly dividend per share (in dollars per share)											CAD 1.00000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series P
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											16,000,000	16,000,000	16,000,000	16,000,000
Preferred stock, value, outstanding | CAD											CAD 400	CAD 400	CAD 400	CAD 400
Initial Yield (as a percent)											4.00%
Yearly dividend per share (in dollars per share)											CAD 1.00000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series R
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											16,000,000	16,000,000	16,000,000	16,000,000
Preferred stock, value, outstanding | CAD											CAD 400	CAD 400	CAD 400	CAD 400
Initial Yield (as a percent)											4.00%
Yearly dividend per share (in dollars per share)											CAD 1.00000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series 1
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											16,000,000	16,000,000	16,000,000	16,000,000
Preferred stock, value, outstanding | CAD											CAD 411	CAD 411	CAD 411	CAD 411
Initial Yield (as a percent)											4.00%
Yearly dividend per share (in dollars per share) | $ / shares												CAD 1.000
Per Share Base Redemption Value (in dollars per shares) | $ / shares																25
Preference shares | Preference Shares, Series 3
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											24,000,000	24,000,000	24,000,000	24,000,000
Preferred stock, value, outstanding | CAD											CAD 600	CAD 600	CAD 600	CAD 600
Initial Yield (as a percent)											4.00%
Yearly dividend per share (in dollars per share)											CAD 1.00000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series 5
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											8,000,000	8,000,000	8,000,000	8,000,000
Preferred stock, value, outstanding | CAD											CAD 206	CAD 206	CAD 206	CAD 206
Initial Yield (as a percent)											4.40%
Yearly dividend per share (in dollars per share) | $ / shares												CAD 1.100
Per Share Base Redemption Value (in dollars per shares) | $ / shares																$ 25
Preference shares | Preference Shares, Series 7
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											10,000,000	10,000,000	10,000,000	10,000,000
Preferred stock, value, outstanding | CAD											CAD 250	CAD 250	CAD 250	CAD 250
Initial Yield (as a percent)											4.40%
Yearly dividend per share (in dollars per share)											CAD 1.10000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series 9
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											11,000,000	11,000,000	11,000,000	11,000,000
Preferred stock, value, outstanding | CAD											CAD 275	CAD 275	CAD 275	CAD 275
Initial Yield (as a percent)											4.40%
Yearly dividend per share (in dollars per share)											CAD 1.10000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series 11
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											20,000,000	20,000,000	20,000,000	20,000,000
Preferred stock, value, outstanding | CAD											CAD 500	CAD 500	CAD 500	CAD 500
Initial Yield (as a percent)											4.40%
Yearly dividend per share (in dollars per share)											CAD 1.10000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series 13
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											14,000,000	14,000,000	14,000,000	14,000,000
Preferred stock, value, outstanding | CAD											CAD 350	CAD 350	CAD 350	CAD 350
Initial Yield (as a percent)											4.40%
Yearly dividend per share (in dollars per share)											CAD 1.10000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series 15
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											11,000,000	11,000,000	11,000,000	11,000,000
Preferred stock, value, outstanding | CAD											CAD 275	CAD 275	CAD 275	CAD 275
Initial Yield (as a percent)											4.40%
Yearly dividend per share (in dollars per share)											CAD 1.10000
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Preference shares | Preference Shares, Series 17
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											30,000,000	30,000,000	30,000,000	0
Preferred stock, value, outstanding | CAD											CAD 750	CAD 750	CAD 750	CAD 0
Initial Yield (as a percent)											5.15%
Yearly dividend per share (in dollars per share)											CAD 1.28750
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Minimum fixed dividend rate upon reset (as a percent)											5.15%
Preference shares | Preference Shares, Series 19
Preference Shares
Preferred stock, shares outstanding (in shares) | shares											20,000,000	20,000,000	0	0
Preferred stock, value, outstanding | CAD											CAD 500	CAD 500	CAD 0	CAD 0
Initial Yield (as a percent)											4.90%
Yearly dividend per share (in dollars per share)											CAD 1.22500
Per Share Base Redemption Value (in dollars per shares)											CAD 25	CAD 25
Minimum fixed dividend rate upon reset (as a percent)											4.90%
Preference shares | Preferred Stock Excluding Series
Preference Shares
Reset term of fixed dividend rate											5 years
Preference shares | Preference Shares, Series E
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.40%
Preference shares | Series G Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.50%
Preference shares | Series I Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.10%
Preference shares | Series O Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.70%
Preference shares | Series Q Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.50%
Preference shares | Series S Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.50%
Preference shares | Series 4 Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.40%
Preference shares | Series 8 Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.60%
Preference shares | Series10 Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.70%
Preference shares | Series12 Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.60%
Preference shares | Series14 Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.70%
Preference shares | Series16 Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											2.70%
Preference shares | Series18 Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											4.10%
Preference shares | Series 20 Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of Canada treasury bill rate (as a percent)											3.20%
Preference shares | Series K Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of US treasury bill rate (as a percent)											3.10%
Preference shares | Series M Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of US treasury bill rate (as a percent)											3.20%
Preference shares | Series 2 Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of US treasury bill rate (as a percent)											3.10%
Preference shares | Series 6 Preferred Stock
Preference Shares
Quarterly floating rate cumulative dividends per share calculation, spread on Government of US treasury bill rate (as a percent)											2.80%
Scenario, Forecast | Preference Shares, Series D
Preference Shares
Yearly dividend per share (in dollars per share)	CAD 0.27875	CAD 0.25000
Convertible Preferred Stock, Shares Issued upon Conversion | shares															1,000,000