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Equity Awards (Tables) | 12 Months Ended
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Dec. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Schedule of Restricted Stock And Restricted Stock Units Activity | Restricted Stock and Restricted Stock Units The table below sets forth the restricted stock and restricted stock units activity for each of the three years in the period ended December 31, 2019: ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Weighted-
​ | ​ | Number of | ​ | average
​ | ​ | shares or units | ​ | grant price
Balance, December 31, 2016 |  | 727,100 | ​ | $ | 97.53
Grants |  | 75,140 | ​ |  | 82.18
Vested shares |  | (84,908) | ​ |  | 66.60
Forfeitures |  | (1,204) | ​ |  | 62.28
Balance, December 31, 2017 |  | 716,128 | ​ |  | 99.65
Grants |  | 159,005 | ​ |  | 93.30
Awards assumed through acquisition of CEC |  | 136,324 | ​ |  | 118.29
Vested shares |  | (236,164) | ​ |  | 76.78
Forfeitures |  | (37,343) | ​ |  | 83.69
Balance, December 31, 2018 |  | 737,950 | ​ |  | 114.43
Grants |  | 158,748 | ​ |  | 128.87
Vested shares |  | (393,588) | ​ |  | 141.75
Forfeitures |  | (34,160) | ​ |  | 79.02
Balance, December 31, 2019 |  | 468,950 | ​ | $ | 98.98
​
Schedule of Stock Option Activity and Other Stock Option Information | Stock Options The table below sets forth the stock option activity and other stock option information for each of the three years in the period ended December 31, 2019: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ | ​ |  | Weighted- |  | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | average | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted- | ​ | remaining | ​ | Aggregate
​ | ​ | Number of | ​ | average | ​ | contractual | ​ | intrinsic value(1)
​ | ​ | shares | ​ | exercise price | ​ | life (years) | ​ | (in thousands)
Balance, December 31, 2016 |  | 100,000 | ​ | $ | 51.95 |  | 4.1 | ​ | $ | 2,868
Grants |  | — | ​ |  | — | ​ | ​ | ​ | ​ | ​
Exercises |  | — | ​ |  | — | ​ | ​ | ​ | ​ | ​
Forfeitures |  | — | ​ |  | — | ​ | ​ | ​ | ​ | ​
Balance, December 31, 2017 |  | 100,000 | ​ |  | 51.95 |  | 3.1 | ​ |  | 3,763
Grants |  | — | ​ |  | — | ​ | ​ | ​ | ​ | ​
Awards assumed through acquisition of CEC |  | 319,846 | ​ |  | 66.98 | ​ | ​ | ​ | ​ | ​
Exercises |  | (162,831) | ​ |  | 58.11 | ​ | ​ | ​ | ​ | ​
Forfeitures |  | (769) | ​ |  | 51.96 | ​ | ​ | ​ | ​ | ​
Balance, December 31, 2018 |  | 256,246 | ​ |  | 66.80 |  | 7.0 | ​ |  | 11,947
Grants |  | — | ​ |  | — | ​ | ​ | ​ | ​ | ​
Exercises |  | (208,114) | ​ |  | 67.61 | ​ | ​ | ​ | ​ | ​
Forfeitures/Expirations |  | (2,036) | ​ |  | 58.38 | ​ | ​ | ​ | ​ | ​
Balance, December 31, 2019 |  | 46,096 | ​ | $ | 63.49 |  | 5.2 | ​ | $ | 4,398
Exercisable, December 31, 2019 |  | 24,246 | ​ | $ | 60.27 |  | 4.0 | ​ | $ | 2,391
(1) | The aggregate intrinsic value in the table above represents the total pre-tax intrinsic value (the difference between the Company’s closing stock price on the respective trading day and the exercise price, multiplied by the number of in-the-money options) that would have been received by the option holder had all options been exercised on the respective trading day. The amount of intrinsic value will change based on the fair market value of the Company’s common stock.
​
Schedule of Stock-based Compensation Expense | The following table reflects the amount of stock-based compensation expense recorded in each of the expense line items for the years ended December 31, 2017, 2018, and 2019 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2017 |  | 2018 |  | 2019
Instructional and support costs | ​ | $ | 1,943 | ​ | $ | 2,588 | ​ | $ | 3,823
General and administration | ​ |  | 9,684 | ​ |  | 10,702 | ​ |  | 7,970
Merger and integration costs | ​ |  | — | ​ |  | 2,242 | ​ |  | 367
Stock-based compensation expense included in operating expense | ​ |  | 11,627 | ​ |  | 15,532 | ​ |  | 12,160
Tax benefit | ​ |  | 4,593 | ​ |  | 3,922 | ​ |  | 3,126
Stock-based compensation expense, net of tax | ​ | $ | 7,034 | ​ | $ | 11,610 | ​ | $ | 9,034
​

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