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Share Capital and Earnings Per Share - Summary of Further Information on Preferred Shares Outstanding (Parenthetical) (Detail) - CAD / shares
 12 Months Ended
Sep. 19, 2017
Mar. 19, 2017
Dec. 31, 2017
Class 1, Series 4 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Annual dividend rate     Floating
Class 1, Series 4 Preferred shares [Member] | Major ordinary share transactions [Member]      
Disclosure of classes of share capital [Line Items]      
Basis spread on dividend rate     1.41%
Floating dividend rate, description     Three month Government of Canada Treasury bill yield plus 1.41%.
Class 1, Series 4 Preferred shares [Member] | If redeemed on June 19, 2021 [Member]      
Disclosure of classes of share capital [Line Items]      
Price per share     CAD 25.00
Class 1, Series 4 Preferred shares [Member] | If redeemed after June 19, 2021 [Member]      
Disclosure of classes of share capital [Line Items]      
Price per share     CAD 25.50
Class 1, Series 3 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     1.41%
Class 1, Series 5 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     2.90%
Class 1, Series 7 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     3.13%
Annual dividend rate   4.312%  
Class 1, Series 9 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     2.86%
Annual dividend rate 4.351%    
Class 1, Series 11 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     2.61%
Class 1, Series 13 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     2.22%
Class 1, Series 15 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     2.16%
Class 1, Series 17 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     2.36%
Class 1, Series 19 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     2.30%
Class 1, Series 21 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     4.97%
Class 1, Series 23 Preferred shares [Member]      
Disclosure of classes of share capital [Line Items]      
Specified yield percentage     3.83%