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Summary of Significant Accounting Policies (Cash, Short-term Investments and Receivables) (Narrative) (Details) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Cash and Cash Equivalents [Abstract]    
Short-term Investments $ 43 $ 222
Investment Interest Rate 2.00% 1.40%