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STOCK-BASED COMPENSATION (Tables) | 6 Months Ended
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Jun. 30, 2021
STOCK-BASED COMPENSATION |
Schedule of weighted-average assumptions used to estimate the fair value of options granted | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three-Months Ended June 30, | ​ | Six-Months Ended June 30, | ​
​ |  | 2021 |  | 2020* |  | 2021 |  | 2020 | ​
Dividend yield | ​ | 0.0 | % | — | ​ | 0.0 | % | 0.0 | %
Expected volatility | ​ | 29.0 | % | — | ​ | 28.9 | % | 30.5 | %
Risk-free interest rate | ​ | 0.9 | % | — | ​ | 0.8 | % | 0.7 | %
Expected term | ​ | 5.9 | years | — | ​ | 5.8 | years | 5.8 | years
Summary of activities with respect to its stock option plans | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | Weighted- | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted- | ​ | Average | ​ | ​ | ​
​ | ​ | ​ | ​ | Average | ​ | Remaining | ​ | ​ | ​
​ | ​ | Number of | ​ | Exercise | ​ | Contractual | ​ | Aggregate
​ | ​ | Shares (in | ​ | Price Per | ​ | Term (in | ​ | Intrinsic
Options |  | thousands) |  | Share |  | years) |  | Value
Outstanding at January 1, 2021 |  | 13,973 | ​ | $ | 44.93 |  | 5.7 | ​ | $ | 664,432
Granted 01/01/21 - 03/31/21 |  | 1,015 | ​ | $ | 88.95 | ​ | ​ | ​ | ​ | ​
Granted 04/01/21 - 06/30/21 |  | 13 | ​ | $ | 91.36 | ​ | ​ | ​ | ​ | ​
Exercised |  | (570) | ​ | $ | 42.95 | ​ | ​ | ​ | ​ | ​
Cancelled or forfeited |  | (57) | ​ | $ | 61.56 | ​ | ​ | ​ | ​ | ​
Outstanding at June 30, 2021 |  | 14,374 | ​ | $ | 48.09 |  | 5.6 | ​ | $ | 621,788
Vested and expected to vest in the future at June 30, 2021 |  | 14,058 | ​ | $ | 47.61 |  | 5.5 | ​ | $ | 614,822
Exercisable at June 30, 2021 |  | 9,682 | ​ | $ | 40.31 |  | 4.5 | ​ | $ | 494,110
Summary of activities with respect to non-vested restricted stock units | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | Number of | ​ | Average
​ | ​ | Shares (in | ​ | Grant-Date
​ |  | thousands) |  | Fair Value
Non-vested at January 1, 2021 | ​ | 947 | ​ | $ | 60.52
Granted 01/01/21 - 03/31/211 | ​ | 304 | ​ | $ | 86.28
Granted 04/01/21 - 06/30/21 | ​ | 14 | ​ | $ | 92.14
Vested | ​ | (344) | ​ | $ | 62.27
Forfeited/cancelled | ​ | (4) | ​ | $ | 60.15
Non-vested at June 30, 2021 | ​ | 917 | ​ | $ | 68.89
​ 1The grant activity for performance share units is recorded based on the target performance level earning 100% of target performance share units. The actual number of performance share units earned could range from 0% to 200% of target depending on the achievement of pre-established performance goals.

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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=120381028&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=120381028&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=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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