Segmented Information (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Total Revenues	$ 1,210	$ 861	$ 1,083	$ 1,289	$ 822	$ 979	$ 364	$ 753	$ 4,443	$ 2,918	$ 4,422
First Major Customer Accounting For More Than 10 Percent of Revenues [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total Revenues									709	434	447
Second Major Customer Accounting For More Than 10 Percent of Revenues [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total Revenues									$ 412	$ 343	$ 414
Major Customers Accounting For More Than 10 Percent of Revenues [member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Major Customers									2	2	2
Canada [Member] | Major Customers Accounting For More Than 10 Percent of Revenues [member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total Revenues									$ 144
Canada [Member] | Customers Outside of Canada [member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total Revenues									64	$ 50	$ 153
United States [Member] | Major Customers Accounting For More Than 10 Percent of Revenues [member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Total Revenues									$ 977