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Equity Incentive Plan (Tables) | 9 Months Ended
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Sep. 30, 2021
Equity Incentive Plan |
Schedule of restricted share activity | Restricted stock 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 |  | 83 | ​ | $ | 65.10
Restricted stock vested | ​ | (59) | ​ | $ | 54.16
Restricted stock forfeited |  | (24) | ​ | $ | 63.88
​ | ​ | ​ | ​ | ​ | ​
Unvested restricted stock at September 30, 2021 |  | 175 | ​ | $ | 64.40
Schedule of valuation assumptions used | During the years ended December 31, 2021, 2020 and 2019 the following assumptions were used: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Nine Months Ended September 30, | ​ | ​ | 2021 | ​ | 2020 | ​ | 2019
Expected term (years) | ​ | ​ | 2.9 | ​ | ​ | 2.9 | ​ | ​ | 2.9 | ​
Volatility | ​ | ​ | 33.9 | % | ​ | 18.4 | % | ​ | 19.7 | %
Risk-free rate | ​ | ​ | 0.2 | % | ​ | 1.3 | % | ​ | 2.5 | %
Schedule of performance share and unit activity | Performance share and unit 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 |  | 87 | ​ | $ | 69.61
​ | ​ | ​ | ​ | ​ | ​
Performance units granted |  | 43 | ​ | $ | 63.42
Performance units and shares at September 30, 2021 - three-year performance period completed |  | (31) | ​ | $ | 47.73
Performance units and shares forfeited |  | (21) | ​ | $ | 68.79
​ | ​ | ​ | ​ | ​ | ​
Performance units and shares at September 30, 2021 - three-year performance period to be completed | ​ | 78 | ​ | $ | 63.35
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 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 | ​ | $ | 47.73
Shares vested | ​ | (16) | ​ | $ | 47.73
Shares forfeited |  | (4) | ​ | $ | 47.73
​ | ​ | ​ | ​ | ​ | ​
Performance shares - three-year performance period completed but not yet vested September 30, 2021 | ​ | 27 | ​ | $ | 47.73
​ | ​ | ​ | ​ | ​ | ​
(1)Performance shares granted in 2018 for which the three-year performance period was completed in 2021 paid out at the 150% maximum 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 -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + 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=120381028&loc=d3e5070-113901 + 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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