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Equity - Additional Information (Detail)
 3 Months Ended 12 Months Ended
Dec. 31, 2018
CAD ($)
Sep. 30, 2018
CAD ($)
Jun. 30, 2018
CAD ($)
Mar. 31, 2018
CAD ($)
Dec. 31, 2017
CAD ($)
Sep. 30, 2017
CAD ($)
Jun. 30, 2017
CAD ($)
Dec. 31, 2018
CAD ($)
Vote / shares
shares
Dec. 31, 2017
CAD ($)
shares
Disclosure of terms and conditions of share-based payment arrangement [line items]                  
Share option, granted               1,575,000 2,011,000
Weighted average exercise price | $               $ 37.44 $ 27.79
Average share price | $               32.55 27.86
Total share option compensation expense | $               $ 17,000,000 $ 17,000,000
Dilutive shares were not included in the diluted earnings per share | shares               5,458,816 4,240,949
Dividends declared and paid per share | $ $ 0.15 $ 0.05 $ 0.05 $ 0.05 $ 0.45 $ 0.05 $ 0.10    
Deferred share units (DSUs), restricted share units (RSUs), performance share units (PSUs) and performance deferred share units (PDSUs) [member]                  
Disclosure of terms and conditions of share-based payment arrangement [line items]                  
Description of vesting requirements for share-based payment arrangement               PSUs granted prior to 2017 vest in a percentage of the original grant varying from 0% to 200% based on our total shareholder return ranking compared to a group of specified companies. PSUs issued in 2018 and 2017 vest in a percentage from 0% to 200% based on both relative total shareholder return and a calculation based on the change in EBITDA over the vesting period divided by the change in a weighted commodity price index. Once vested, PSUs entitle the holder to a cash payment equal to the weighted average trading price of one Class B subordinate voting share on the Toronto Stock Exchange over either 10 or 20 consecutive trading days prior to vesting, depending on the date issued  
Share based compensation expense | $               $ 42,000,000 $ 108,000,000
Total liability and intrinsic value for vested Units | $ 103,000,000       185,000,000     $ 103,000,000 $ 185,000,000
Top of range [member] | Performance share units (PSUs) [member]                  
Disclosure of terms and conditions of share-based payment arrangement [line items]                  
Vesting percentage               200.00% 200.00%
Bottom of range [member] | Performance share units (PSUs) [member]                  
Disclosure of terms and conditions of share-based payment arrangement [line items]                  
Vesting percentage               0.00% 0.00%
Class A Common Shares [member]                  
Disclosure of terms and conditions of share-based payment arrangement [line items]                  
Number of vote per share | Vote / shares               100  
Class A share conversion | shares               (9,000) (1,576,000)
Percentage of voting equity interests before conversion                 62.20%
Percentage of voting equity interests after conversion                 57.70%
Class B Subordinate Voting Shares [member]                  
Disclosure of terms and conditions of share-based payment arrangement [line items]                  
Number of vote per share | Vote / shares               1  
Class A share conversion | shares               9,000 1,576,000
Purchase of voting shares under normal course issuer bid | shares               6,539,558  
Shares repurchased, cancelled | shares               6,299,558  
Shares repurchased, pending for cancellation | shares               240,000  
Class B Subordinate Voting Shares [member] | Stock Options [member]                  
Disclosure of terms and conditions of share-based payment arrangement [line items]                  
Share option, granted | shares               28,000,000  
Share option, available for grant | shares               3,443,007  
Share option, granted               1,575,355  
Weighted average exercise price | $               $ 37.44  
Share option granted, vesting period               3 years  
Share option granted, term               10 years  
Weighted average fair value of share option | $ $ 11.10       $ 8.32     $ 11.10 $ 8.32