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Share-Based Compensation and Benefit Plans (Tables) | 3 Months Ended
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Mar. 31, 2020
Restricted stock [Member] |
Share-Based Compensation and Benefit Plans |
Summary of activity of share-based compensation and benefit plans | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | For the Three Months Ended
​ | ​ | March 31,
​ |  | 2020 |  | 2019
Compensation expense for shares issued under the ESPP | ​ | $ | 646 | ​ | $ | 577
Income tax benefit from compensation expense related to shares issued under the ESPP | ​ | ​ | 169 | ​ | ​ | 142
Compensation expense for restricted shares awarded | ​ | ​ | 351 | ​ | ​ | 339
Income tax benefit from compensation expense related to restricted awards | ​ | $ | 92 | ​ | $ | 84
Stock option [Member] |
Share-Based Compensation and Benefit Plans |
Summary of stock options | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Shares | ​ | Weighted- Average
​ | ​ | (in thousands) | ​ | Exercise Price
Outstanding at December 31, 2019 |  | 1,635 | ​ | $ | 218.10
Granted |  | 137 | ​ |  | 387.30
Exercised |  | (52) | ​ |  | 116.78
Forfeited or expired |  | (7) | ​ |  | 258.79
Outstanding at March 31, 2020 |  | 1,713 | ​ | $ | 234.59
Exercisable at March 31, 2020 |  | 1,105 | ​ | $ | 183.34
Black-Scholes option pricing model | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | March 31,
​ |  | 2020 | ​ | 2019
Risk free interest rate |  | 1.02 | % | ​ | 2.50 | %
Expected life |  | 6.4 | Years | ​ | 6.2 | Years
Expected volatility |  | 25.5 | % | ​ | 25.0 | %
Expected dividend yield |  | — | % | ​ | — | %
Summary of activity of share-based compensation and benefit plans | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | For the Three Months Ended
​ | ​ | March 31,
​ |  | 2020 |  | 2019
Compensation expense for stock options awarded | ​ | $ | 4,878 | ​ | $ | 4,508
Income tax benefit from compensation expense related to stock options | ​ |  | 1,276 | ​ |  | 1,112
Weighted-average grant-date fair value of options awarded | ​ | $ | 107.26 | ​ | $ | 108.74

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- Definition Tabular disclosure of share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + 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 (d) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=116856206&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=116856206&loc=d3e5070-113901 + Details
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