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Equity Incentive Plan (Tables) | 6 Months Ended
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Jun. 30, 2024
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, 2023 |  | 194 | ​ | $ | 68.85
​ | ​ | ​ | ​ | ​ | ​
Restricted stock granted |  | 101 | ​ | $ | 57.51
Restricted stock vested | ​ | (64) | ​ | $ | 69.37
Restricted stock forfeited |  | (4) | ​ | $ | 64.80
Unvested restricted stock at June 30, 2024 |  | 227 | ​ | $ | 63.74
Schedule of valuation assumptions used | The following assumptions were used when determining the grant date fair value:
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | 2024 | ​ | 2023 | ​ | 2022
Expected term (years) | ​ | ​ | 2.9 | ​ | ​ | 2.9 | ​ | ​ | 2.9 | ​
Volatility | ​ | ​ | 20.0 | % | ​ | 23.6 | % | ​ | 33.5 | %
Risk-free rate | ​ | ​ | 4.5 | % | ​ | 4.4 | % | ​ | 1.8 | %
Schedule of performance share and unit activity | Performance units activity is summarized as follows:
​ | ​ | ​ | ​ | ​ | ​
​ |  | Target Number |  | Weighted Average
​ | ​ | of Awards | ​ | Grant Date
​ | ​ | (in thousands) | ​ | Fair Value
Performance units at December 31, 2023 - three-year performance period to be completed | ​ | 111 | ​ | $ | 72.14
​ | ​ | ​ | ​ | ​ | ​
Performance units granted | ​ | 77 | ​ | $ | 59.16
Performance shares - three-year performance period completed | ​ | (31) | ​ | $ | 63.42
Performance units at June 30, 2024 - three-year performance period to be completed | ​ | 157 | ​ | $ | 67.50
​
​ | ​ | ​ | ​ | ​
​ | Shares |  | Weighted Average
​ | Outstanding | ​ | Grant Date
​ | (in thousands) | ​ | Fair Value
Performance units and shares - three-year performance period completed but not yet vested at December 31, 2023 | 31 | ​ | $ | 83.40
​ | ​ | ​ | ​ | ​
Shares earned at completion of three-year performance period (1) | 23 | ​ | $ | 63.42
Shares vested | (28) | ​ | $ | 75.18
Performance units and shares - three-year performance period completed but not yet vested at June 30, 2024 | 26 | ​ | $ | 74.58
​ | ​ | ​ | ​ | ​
​ (1)Performance units granted in 2021 for which the three-year performance period was completed in 2024 were earned at the 76% performance level.

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- Definition Tabular disclosure of the changes in outstanding nonvested performance-based units. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + 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 -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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