BASIS OF PRESENTATION - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cash and cash equivalents		$ 457		$ 480
Increase (reduction) to net cash provided by financing activities		3,399	$ (1,678)
Net cash used in investing activities		$ (3,139)	(5,829)
Scenario, Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cash and cash equivalents				$ 600
Increase (reduction) to net cash provided by financing activities	$ 941		400
Merger Transaction | Scenario, Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Increase (reduction) to net cash provided by financing activities			67
Net cash used in investing activities			$ 67