Income Taxes Effective Income Tax Rate (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Statutory income tax	$ 37.2	$ 19.2	$ 25.3
Effective Income Tax Rate Reconciliation, Deduction, Amount	0.0	0.0	(2.0)
Valuation allowance	(0.2)	0.0	(2.0)
Tax impact on the future tax rate change	0.0	0.8	1.9
Inter-corporate charges not tax deducted	2.4	2.2	(0.3)
Other	(1.6)	(1.4)	0.5
Total tax provision	$ 37.8	$ 20.8	$ 23.4
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory income tax	27.00%	27.00%	27.00%
Effective Income Tax Rate Reconciliation, Deduction, Percent	0.00%	0.00%	(2.10%)
Valuation allowance	(0.10%)	0.00%	(2.10%)
Tax impact on the future tax rate change	0.00%	1.10%	2.00%
Inter-corporate charges not tax deducted	1.70%	3.10%	(0.30%)
Other	(1.20%)	(2.00%)	0.50%
Total	27.40%	29.20%	25.00%