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Shareholders' capital - Common Shares (Detail) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Common Shares Rollforward    
Beginning balance (in shares) 274,087,018 255,869,419
Public offering and subscription receipts (in shares) 43,470,000 12,938,457
Debt conversion, shares issued (in shares) 108,370,081 0
Dividend reinvestment plan (in shares) 3,905,848 2,322,618
Exercise of share-based awards 1,932,988 2,956,524
Ending balance (in shares) 431,765,935 274,087,018