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Stock option and award plan: (Tables) | 12 Months Ended
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Dec. 31, 2024
Stock option and award plan: |
Schedule of assumptions used for determining the fair value of options granted | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Years Ended |
​ | ​ | December 31, | ​
Black-Scholes Assumptions |  | 2024 |  | 2023 |  | 2022 |
Dividend yield |  | 6.0 | % | 5.8 | % | 5.9 | %
Expected volatility |  | 34.9 | % | 33.4 | % | 33.1 | %
Risk-free interest rate |  | 4.1 | % | 3.8 | % | 3.0 | %
Expected life of the option term (in years) |  | 4.1 | ​ | 4.1 | ​ | 4.1 | ​
Schedule of stock option activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | Number of |  | Weighted-Average
​ |  | Options |  | Exercise Price
Outstanding at December 31, 2023 |  | 198,775 | ​ | $ | 61.97
Granted |  | 120,784 | ​ | $ | 70.21
Cancelled and expired |  | (62,138) | ​ | $ | 66.05
Exercised—intrinsic value $0.5 million; cash received $1.2 million |  | (36,465) | ​ | $ | 60.45
Outstanding at December 31, 2024—$2.7 million intrinsic value and 7.7 years weighted-average remaining contractual term |  | 220,956 | ​ | $ | 65.58
Exercisable at December 31, 2024—$1.5 million intrinsic value and 6.1 years weighted-average remaining contractual term |  | 95,330 | ​ | $ | 62.15
Expected to vest—$2.3 million intrinsic value and 7.4 years weighted-average remaining contractual term |  | 183,864 | ​ | $ | 64.94
Schedule of non-vested restricted stock awards | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted-Average
​ | ​ | ​ | ​ | Grant Date
Non-vested awards |  | Shares |  | Fair Value
Non-vested at December 31, 2023 |  | 1,261,623 | ​ | $ | 62.89
Granted |  | 672,098 | ​ | $ | 68.58
Vested |  | (315,559) | ​ | $ | 66.11
Forfeited |  | (128,885) | ​ | $ | 66.66
Non-vested at December 31, 2024 |  | 1,489,277 | ​ | $ | 64.45
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Years Ended
​ |  | December 31,
Additional Award Plan Information – Related to Stock Options & Restricted Stock (thousands) |  | 2024 |  | 2023 |  | 2022
Equity-based compensation expense | ​ | $ | 25,738 | ​ | $ | 26,924 | ​ | $ | 24,439
Income tax benefit related to stock options and restricted stock | ​ |  | 2,469 | ​ |  | 3,307 | ​ |  | 2,489
Capitalized compensation expense related to stock options and restricted stock | ​ |  | 3,100 | ​ |  | 3,541 | ​ |  | 2,277
Intrinsic value of stock options exercised | ​ |  | 573 | ​ |  | 456 | ​ |  | 305
Fair value of shares of restricted stock vested | ​ |  | 22,489 | ​ |  | 30,113 | ​ |  | 25,792

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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 -Topic 718 -SubTopic 10 -Subparagraph (f)(2) -Name Accounting Standards Codification -Paragraph 2 -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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