Changes in Operating Assets and Liabilities (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Accounts receivable	$ 25.0	$ (3.8)	$ 25.9
Inventories	(0.2)	(0.7)	(1.5)
Other current assets	(4.0)	4.1	(3.2)
Deferred charges and other assets	(5.6)	(12.4)	(4.2)
Accounts payable	(48.0)	(21.3)	35.3
Accrued taxes	303.8	10.2	(0.5)
Other current liabilities	3.6	(55.5)	3.5
Other deferred credits	62.4	(15.0)	(82.8)
Increase (Decrease) in Operating Capital	$ 337.0	$ (94.4)	$ (27.5)