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financial instruments - Accounts receivable (Details) - Customer accounts receivable - CAD ($)
$ in Millions
 12 Months Ended
Jan. 01, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2017
Credit risk            
Weighted average age of receivables 26 days 30 days 26 days      
Weighted average age of past-due receivables 61 days 56 days 60 days      
Receivable account balances $ 1,154 $ 1,169 $ 1,169 $ 1,210 $ 1,169 $ 1,154
Written off amounts charged to allowance for doubtful accounts still subject to enforcement       353 298 231
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   (1,169)        
Balance, end of period (1,154) (1,210) (1,169)      
Less than 30 days past billing date            
Credit risk            
Receivable account balances 888 895 895 749 895 888
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   (895)        
Balance, end of period (888) (749) (895)      
30-60 days past billing date            
Credit risk            
Receivable account balances 176 177 177 344 177 176
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   (177)        
Balance, end of period (176) (344) (177)      
61-90 days past billing date            
Credit risk            
Receivable account balances 35 52 52 72 52 35
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   (52)        
Balance, end of period (35) (72) (52)      
More than 90 days past billing date            
Credit risk            
Receivable account balances 55 45 45 45 45 55
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   (45)        
Balance, end of period (55) (45) (45)      
Cost/Gross amount            
Credit risk            
Receivable account balances 1,208 1,212 1,212 1,263 1,212 1,208
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   (1,212)        
Balance, end of period (1,208) (1,263) (1,212)      
Cost/Gross amount | Less than 30 days past billing date            
Credit risk            
Receivable account balances 899 905 905 762 905 899
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   (905)        
Balance, end of period (899) (762) (905)      
Cost/Gross amount | 30-60 days past billing date            
Credit risk            
Receivable account balances 185 185 185 354 185 185
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   (185)        
Balance, end of period (185) (354) (185)      
Cost/Gross amount | 61-90 days past billing date            
Credit risk            
Receivable account balances 44 60 60 80 60 44
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   (60)        
Balance, end of period (44) (80) (60)      
Cost/Gross amount | More than 90 days past billing date            
Credit risk            
Receivable account balances 80 62 62 67 62 80
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   (62)        
Balance, end of period (80) (67) (62)      
Allowance for doubtful accounts            
Credit risk            
Receivable account balances (54) (43) (54) (53) (43) (54)
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period 54 43 54      
Additions (doubtful accounts expense)   56 54      
Accounts written off, net of recoveries   (55) (66)      
Other   9 1      
Balance, end of period 54 53 43      
Allowance for doubtful accounts | Less than 30 days past billing date            
Credit risk            
Receivable account balances (11) (10) (10) (13) (10) (11)
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   10        
Balance, end of period 11 13 10      
Allowance for doubtful accounts | 30-60 days past billing date            
Credit risk            
Receivable account balances (9) (8) (8) (10) (8) (9)
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   8        
Balance, end of period 9 10 8      
Allowance for doubtful accounts | 61-90 days past billing date            
Credit risk            
Receivable account balances (9) (8) (8) (8) (8) (9)
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   8        
Balance, end of period 9 8 8      
Allowance for doubtful accounts | More than 90 days past billing date            
Credit risk            
Receivable account balances (25) (17) (17) $ (22) $ (17) $ (25)
Summary of activity related to allowance for doubtful accounts            
Balance, beginning of period   17        
Balance, end of period $ 25 $ 22 $ 17