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Shareholders' capital (Tables) | 6 Months Ended
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Jun. 30, 2019
Equity [Abstract] |
Number of Common Shares | Number of common shares:
|  | Six Months Ended June 30
|  | 2019 |  | 2018
Common shares, beginning of period |  | 488,851,433 |  |  | 431,765,935 |
Public offering |  | 509,431 |  |  | 37,505,274 |
Dividend reinvestment plan |  | 3,225,749 |  |  | 2,532,767 |
Exercise of share-based awards (c) |  | 886,931 |  |  | 352,800 |
Conversion of convertible debentures |  | 11,883 |  |  | 38,138 |
Common shares, end of period |  | 493,485,427 |  |  | 472,194,914 |
Fair Value of Share Options Granted | The following assumptions were used in determining the fair value of share options granted:
| 2019 |
Risk-free interest rate | 1.9 | %
Expected volatility | 20 | %
Expected dividend yield | 4.3 | %
Expected life | 5.50 years |
Weighted average grant date fair value per option | C$ | 1.66 |

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- Definition Tabular disclosure of the change in common stock outstanding. + References No definition available. + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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