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Financial Instrument Risk Management - Narrative (Details) - CAD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of financial assets [line items]    
Percent of debt security portfolio considered investment grade 99.00% 98.00%
Available-for-sale    
Disclosure of financial assets [line items]    
Impairment loss on available-for-sale assets $ 12,000,000 $ 7,000,000
Debt securities | Available-for-sale    
Disclosure of financial assets [line items]    
Reversal of impairment $ 0 $ 0