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Equity Incentive Plan (Tables) | 12 Months Ended
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Dec. 31, 2020
Equity Incentive Plan |
Schedule of restricted share activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | Shares |  | Weighted Average
​ | ​ | Outstanding | ​ | Grant Date
​ | ​ | (in thousands) | ​ | Fair Value
Unvested restricted stock at December 31, 2017 |  | 227 | ​ | $ | 39.47
​ | ​ | ​ | ​ | ​ | ​
Restricted stock granted |  | 57 | ​ | $ | 48.85
Restricted stock vested |  | (71) | ​ | $ | 36.06
Restricted stock forfeited |  | (1) | ​ | $ | 48.28
​ | ​ | ​ | ​ | ​ | ​
Unvested restricted stock at December 31, 2018 |  | 212 | ​ | $ | 42.74
​ | ​ | ​ | ​ | ​ | ​
Restricted stock granted |  | 54 | ​ | $ | 65.85
Restricted stock vested |  | (70) | ​ | $ | 39.55
Restricted stock forfeited |  | (2) | ​ | $ | 54.08
​ | ​ | ​ | ​ | ​ | ​
Unvested restricted stock at December 31, 2019 |  | 194 | ​ | $ | 50.71
​ | ​ | ​ | ​ | ​ | ​
Restricted stock granted |  | 52 | ​ | $ | 78.43
Restricted stock vested | ​ | (68) | ​ | $ | 45.78
Restricted stock forfeited |  | (3) | ​ | $ | 63.80
​ | ​ | ​ | ​ | ​ | ​
Unvested restricted stock at December 31, 2020 |  | 175 | ​ | $ | 60.53
Schedule of valuation assumptions used | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Year Ended December 31, | ​ | ​ | 2020 | ​ | 2019 | ​ | 2018
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Expected term (years) | ​ | ​ | 2.9 | ​ | ​ | 2.9 | ​ | ​ | 2.9 | ​
Volatility | ​ | ​ | 18.4 | % | ​ | 19.7 | % | ​ | 19.1 | %
Dividend yield | ​ | ​ | 2.9 | % | ​ | 3.4 | % | ​ | 4.4 | %
Risk-free rate | ​ | ​ | 1.3 | % | ​ | 2.5 | % | ​ | 2.4 | %
Schedule of performance share and unit activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | Target Number |  | Weighted Average
​ | ​ | of Awards | ​ | Grant Date
​ | ​ | (in thousands) | ​ | Fair Value
Performance shares at December 31, 2017 |  | — | ​ | $ | —
​ | ​ | ​ | ​ | ​ | ​
Performance shares granted | ​ | 31 | ​ | $ | 55.29
​ | ​ | ​ | ​ | ​ | ​
Performance shares at December 31, 2018 |  | 31 | ​ | $ | 55.29
​ | ​ | ​ | ​ | ​ | ​
Performance shares granted | ​ | 30 | ​ | $ | 66.96
​ | ​ | ​ | ​ | ​ | ​
Performance units and shares at December 31, 2019 |  | 61 | ​ | $ | 61.04
​ | ​ | ​ | ​ | ​ | ​
Performance units granted |  | 26 | ​ | $ | 90.17
​ | ​ | ​ | ​ | ​ | ​
Performance units and shares at December 31, 2020 | ​ | 87 | ​ | $ | 69.61
​ |  | ​ | ​ | ​ | ​

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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=120381028&loc=d3e5070-113901 + Details
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