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Shareholders' capital - Common Shares (Detail) - shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Common Shares Rollforward    
Beginning balance (in shares) 431,765,935 274,087,018
Public offering and subscription receipts (in shares) 50,041,624 43,470,000
Conversion of convertible debentures (in shares) 56,926 108,370,081
Dividend reinvestment plan (in shares) 5,880,843 3,905,848
Exercise of share-based awards 1,106,105 1,932,988
Ending balance (in shares) 488,851,433 431,765,935