Share-based Compensation and Benefit Plans (Benefit Plans) (Details) - CAD ($) $ in Millions	4 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total cost for DCPP	$ 0.5
Benefit plan losses relates to he Trans Mountain Transactions		$ 1,278.4	$ 0.0	$ 0.0
Other Postretirement Benefits Plan [Member]
Change in benefit obligation
Benefit obligation at August 31, 2018	1.8
Service cost	0.1
Interest cost	0.0
Actuarial loss (gain)	(0.1)
Benefits paid	0.0
Benefit obligation at end of period	1.8	1.8
Presented as follows:
Current benefit liability	0.0	0.0
Non-current benefit liability	(1.8)	(1.8)
Total	(1.8)	(1.8)
Expected future benefit payments:
2019	0.0	0.0
2020	0.0	0.0
2021	0.0	0.0
2022	0.0	0.0
2023	0.1	0.1
2024-2028	$ 0.4	$ 0.4
Assumptions related to benefit obligations:
Discount rate	3.91%	3.91%
Benefit Obligation [Member] | Other Postretirement Benefits Plan [Member]
Assumptions related to benefit costs:
Discount rate		3.76%
Discount Rate For Interest On Benefit Obligations [Member] | Other Postretirement Benefits Plan [Member]
Assumptions related to benefit costs:
Discount rate		3.67%
Discount Rate For Service Cost On Benefit Obligations [Member] | Other Postretirement Benefits Plan [Member]
Assumptions related to benefit costs:
Discount rate		3.78%
Discount Rate For Interest On Service Cost Benefit Obligations [Member] | Other Postretirement Benefits Plan [Member]
Assumptions related to benefit costs:
Discount rate		3.74%
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefit plan losses relates to he Trans Mountain Transactions		$ 36.3