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EMPLOYEE BENEFITS AND STOCK BASED COMPENSATION (Tables) | 12 Months Ended
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Dec. 31, 2020
Employee Benefits and Share-based Compensation, Noncash [Abstract] |
Payment Made for Employee Benefit Plans | The following 401(k) Plan match and other discretionary contributions were made to the Company’s employees, in accordance with the plans described above, in 2020, 2019, and 2018 (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2020 |  | 2019 |  | 2018
401(k) Plan | ​ | $ | 6,265 | ​ | $ | 5,550 | ​ | $ | 4,592
ESOP | ​ |  | 1,000 | ​ |  | 1,163 | ​ |  | 1,005
Cash | ​ |  | 697 | ​ |  | 780 | ​ |  | 1,509
Total | ​ | $ | 7,962 | ​ | $ | 7,493 | ​ | $ | 7,106
Schedule of Recognized Stock-Based Compensation Expense | For the years ended December 31, 2020, 2019, and 2018, the Company recognized stock-based compensation expense, which is included in salaries and benefits expense on the Company’s Consolidated Statements of Income (dollars in thousands, except per share data) as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31,
​ |  | 2020 |  | 2019 |  | ​ | 2018
Stock-based compensation expense | ​ | $ | 9,258 | ​ | $ | 8,332 | ​ | $ | 6,132
Reduction of income tax expense | ​ |  | 1,944 | ​ |  | 1,750 | ​ |  | 1,287
Per share compensation cost | ​ | $ | 0.09 | ​ | $ | 0.08 | ​ | $ | 0.07
Summary of Stock Option Activity | The following table summarizes the stock option activity during the year ended December 31, 2020:
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ | ​ |  | Weighted |  | ​ | ​
​ | ​ | ​ | ​ | Weighted | ​ | Average | ​ | ​ | ​
​ | ​ | Stock | ​ | Average | ​ | Remaining | ​ | Aggregate
​ | ​ | Options | ​ | Exercise | ​ | Contractual | ​ | Intrinsic
​ | ​ | (shares) | ​ | Price | ​ | Life | ​ | Value
Outstanding as of December 31, 2019 |  | 422,138 | ​ | $ | 32.48 |  |  |  | ​ |
Granted |  | — | ​ |  | — |  |  |  | ​ |
Exercised |  | (46,278) | ​ |  | 22.04 |  |  |  | ​ |
Forfeited |  | (4,582) | ​ |  | 31.83 |  |  |  | ​ |
Expired |  | (22,618) | ​ |  | 33.40 |  |  |  | ​ |
Outstanding as of December 31, 2020 |  | 348,660 | ​ |  | 33.81 |  | 1.76 | ​ | $ | 798,340
Exercisable as of December 31, 2020 |  | 317,237 | ​ |  | 34.00 |  | 1.63 | ​ |  | 764,501
Summary of Nonvested Stock Activity | The following table summarizes the restricted stock activity for the year ended December 31, 2020:
​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Weighted
​ | ​ | Number of | ​ | Average
​ | ​ | Shares of | ​ | Grant-Date Fair
​ | ​ | RSAs | ​ | Value
Unvested as of December 31, 2019 |  | 431,642 | ​ | $ | 34.90
Granted |  | 208,514 | ​ |  | 33.63
Net settle for taxes |  | (55,336) | ​ |  | 36.40
Vested |  | (161,445) | ​ |  | 32.09
Forfeited |  | (16,868) | ​ |  | 34.19
Unvested as of December 31, 2020 |  | 406,507 | ​ |  | 35.61
Summary of Performance Stock Activity | The Plan permits the granting of PSUs. PSUs are granted to certain employees at no cost to the recipient and are subject to vesting based on achieving certain performance metrics; the grant of PSUs is subject to approval by the Company’s Compensation Committee at its sole discretion. PSUs may be paid in cash or shares of common stock or a combination thereof. Holders of PSUs have no right to vote the shares represented by the units. In 2020, the PSUs awarded were market based awards with the number of PSUs ultimately earned based on the Company’s total shareholder return as measured over the performance period. ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | Number of |  | Weighted Average
​ | ​ | Shares of | ​ | Grant-
​ | ​ | PSUs | ​ | Date Fair Value
Unvested as of December 31, 2019 |  | 144,709 | ​ | $ | 37.24
Granted |  | 108,302 | ​ |  | 33.99
Net settle for taxes |  | (14,186) | ​ |  | 44.72
Vested |  | (38,922) | ​ |  | 39.80
Forfeited |  | (2,093) | ​ |  | 32.50
Unvested as of December 31, 2020 |  | 197,810 | ​ |  | 34.84
Schedule of Performance Stock Units Valuation Assumptions | During years ended December 31, 2020, 2019 and 2018 PSUs were awarded with a market based component based on total shareholder return. The fair value of each PSU granted is estimated on the date of grant using the Monte Carlo simulation lattice model that uses the assumptions noted in the following table: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2020(5) |  | 2019(5) |  | 2018(5) |
Dividend yield(1) |  | 2.83 | % | 2.57 | % | 2.25 | %
Expected life in years(2) |  | 2.86 |  | 2.86 |  | 2.86 | ​
Expected volatility(3) |  | 24.33 | % | 24.04 | % | 23.47 | %
Risk-free interest rate(4) |  | 1.35 | % | 2.48 | % | 2.38 | %
(1) | Calculated as the ratio of the current dividend paid per the stock price on the date of grant.
(2) | Represents the remaining performance period as of the grant date.
(3) | Based on the historical volatility for the period commensurate with the expected life of the PSUs.
(4) | Based upon the zero-coupon U.S. Treasury rate commensurate with the expected life of the PSUs on the grant date.
(5) | Assumptions disclosed represent those used in the primary annual issuance.
Estimated Unamortized Compensation Expense Recognized in Future | The estimated unamortized compensation expense, net of estimated forfeitures, related to, restricted stock, performance stock and stock options issued and outstanding as of December 31, 2020 that will be recognized in future periods is as follows (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Restricted |  | Performance | ​ | ​ | Stock |  | ​ | ​
​ | ​ | Stock | ​ | Stock | ​ | ​ | Options | ​ | Total
2021 | ​ | $ | 4,997 | ​ | $ | 1,844 | ​ | $ | 67 | ​ | $ | 6,908
2022 | ​ |  | 2,579 | ​ |  | 957 | ​ | ​ | 66 | ​ |  | 3,602
2023 | ​ |  | 434 | ​ |  | — | ​ | ​ | 4 | ​ |  | 438
2024 | ​ |  | 42 | ​ |  | — | ​ | ​ | — | ​ |  | 42
Total | ​ | $ | 8,052 | ​ | $ | 2,801 | ​ | $ | 137 | ​ | $ | 10,990

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