BASIS OF PRESENTATION - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cash and cash equivalents	$ 610		$ 480
Increase (reduction) to net cash provided by financing activities	(1,009)	$ 1,313
Net cash used in investing activities	(2,068)	(3,448)
Scenario, Adjustment
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Cash and cash equivalents	900		$ 600
Increase (reduction) to net cash provided by financing activities		$ (200)
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