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Equity Incentive Plan (Tables) | 9 Months Ended
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Sep. 30, 2020
Equity Incentive Plan |
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] | Restricted stock activity is summarized as follows: ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | Shares |  | Weighted Average
​ | ​ | Outstanding | ​ | Grant Date
​ | ​ | (in thousands) | ​ | Fair Value
Unvested restricted stock at December 31, 2019 |  | 194 | ​ | $ | 50.71
​ | ​ | ​ | ​ | ​ | ​
Restricted stock granted |  | 52 | ​ | $ | 78.43
Restricted stock vested | ​ | (62) | ​ | $ | 45.45
Restricted stock forfeited |  | (3) | ​ | $ | 62.73
​ | ​ | ​ | ​ | ​ | ​
Unvested restricted stock at September 30, 2020 |  | 181 | ​ | $ | 60.26
Schedule of Nonvested Performance-based Units Activity [Table Text Block] | ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | Target Number |  | ​ | ​
​ | ​ | of Awards | ​ | Weighted Average
​ | ​ | (in thousands) | ​ | Grant Date
Performance units and shares at December 31, 2019 |  | 61 | ​ | $ | 56.57
​ | ​ | ​ | ​ | ​ | ​
Performance units granted |  | 26 | ​ | $ | 79.62
​ | ​ | ​ | ​ | ​ | ​
Performance units and shares at September 30, 2020 | ​ | 87 | ​ | $ | 63.35
​ |  | ​ | ​ | ​ | ​

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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 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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