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Equity Incentive Plan (Tables) | 12 Months Ended
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Dec. 31, 2023
Equity Incentive Plan |
Schedule of restricted share activity | Restricted share activity is summarized as follows:
​ | ​ | ​ | ​ | ​ | ​
​ |  | Shares |  | Weighted Average
​ | ​ | Outstanding | ​ | Grant Date
​ | ​ | (in thousands) | ​ | Fair Value
Unvested restricted stock at December 31, 2020 |  | 175 | ​ | $ | 60.53
​ | ​ | ​ | ​ | ​ | ​
Restricted stock granted |  | 87 | ​ | $ | 65.23
Restricted stock vested |  | (64) | ​ | $ | 53.82
Restricted stock forfeited |  | (23) | ​ | $ | 63.88
Unvested restricted stock at December 31, 2021 |  | 175 | ​ | $ | 64.90
​ | ​ | ​ | ​ | ​ | ​
Restricted stock granted |  | 81 | ​ | $ | 63.10
Restricted stock vested |  | (63) | ​ | $ | 60.84
Restricted stock forfeited |  | (10) | ​ | $ | 65.12
Unvested restricted stock at December 31, 2022 |  | 183 | ​ | $ | 65.46
​ | ​ | ​ | ​ | ​ | ​
Restricted stock granted |  | 82 | ​ | $ | 73.15
Restricted stock vested | ​ | (56) | ​ | $ | 63.95
Restricted stock forfeited |  | (15) | ​ | $ | 69.12
Unvested restricted stock at December 31, 2023 |  | 194 | ​ | $ | 68.85
Schedule of valuation assumptions used | The following assumptions were used when determining the grant date fair value:
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | 2023 | ​ | 2022 | ​ | 2021
Expected term (years) | ​ | ​ | 2.9 | ​ | ​ | 2.9 | ​ | ​ | 2.9 | ​
Volatility | ​ | ​ | 23.6 | % | ​ | 33.5 | % | ​ | 33.9 | %
Risk-free rate | ​ | ​ | 4.4 | % | ​ | 1.8 | % | ​ | 0.2 | %
Schedule of performance share and unit activity | Performance unit and share activity is summarized as follows:
​ | ​ | ​ | ​ | ​ | ​
​ |  | Target Number |  | Weighted Average
​ | ​ | of Awards | ​ | Grant Date
​ | ​ | (in thousands) | ​ | Fair Value
Performance units and shares at December 31, 2020 - three-year performance period to be completed |  | 87 | ​ | $ | 69.61
​ | ​ | ​ | ​ | ​ | ​
Performance units granted |  | 43 | ​ | $ | 63.42
Performance shares - three-year performance period completed |  | (31) | ​ | $ | 55.29
Performance units and shares forfeited |  | (21) | ​ | $ | 68.79
Performance units and shares at December 31, 2021 - three-year performance period to be completed | ​ | 78 | ​ | $ | 72.13
​ | ​ | ​ | ​ | ​ | ​
Performance units granted | ​ | 34 | ​ | $ | 68.59
Performance shares - three-year performance period completed | ​ | (27) | ​ | $ | 66.96
Performance units at December 31, 2022 - three-year performance period to be completed | ​ | 85 | ​ | $ | 72.27
​ | ​ | ​ | ​ | ​ | ​
Performance units granted | ​ | 47 | ​ | $ | 80.34
Performance shares - three-year performance period completed | ​ | (21) | ​ | $ | 90.17
Performance units at December 31, 2023 - three-year performance period to be completed | ​ | 111 | ​ | $ | 72.14
​
​ | ​ | ​ | ​ | ​
​ | Shares |  | Weighted Average
​ | Outstanding | ​ | Grant Date
​ | (in thousands) | ​ | Fair Value
Performance shares - three-year performance period completed but not yet vested at December 31, 2020 | — | ​ | $ | —
​ | ​ | ​ | ​ | ​
Shares earned at completion of three-year performance period (1) | 47 | ​ | $ | 55.29
Shares vested | (16) | ​ | $ | 55.29
Shares forfeited | (4) | ​ | $ | 55.29
Performance shares - three-year performance period completed but not yet vested December 31, 2021 | 27 | ​ | $ | 55.29
​ | ​ | ​ | ​ | ​
Shares earned at completion of three-year performance period (2) | 28 | ​ | $ | 66.96
Shares vested | (23) | ​ | $ | 59.91
Performance units and shares - three-year performance period completed but not yet vested at December 31, 2022 | 32 | ​ | $ | 61.91
​ | ​ | ​ | ​ | ​
Shares earned at completion of three-year performance period (3) | 33 | ​ | $ | 90.17
Shares vested | (34) | ​ | $ | 69.73
Performance units and shares - three-year performance period completed but not yet vested at December 31, 2023 | 31 | ​ | $ | 83.40
​ | ​ | ​ | ​ | ​
(1)Performance shares granted in 2018 for which the three-year performance period was completed in 2021 paid out at the 150% performance level (2)Performance units granted in 2019 for which the three-year performance period was completed in 2022 paid out at the 106% performance level (3)Performance units granted in 2020 for which the three-year performance period was completed in 2023 paid out at the 150% performance level

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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 -Topic 718 -SubTopic 10 -Subparagraph (f)(2) -Name Accounting Standards Codification -Paragraph 2 -Section 50 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + Details
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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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