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Stock-Based Compensation Expense | 9 Months Ended
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Sep. 30, 2023
Share-based Payment Arrangement [Abstract] |
Stock-Based Compensation Expense | 12. Stock-Based Compensation Expense. Stock-based compensation expense before income tax expense for the three and nine-month periods ended September 30, 2023 and 2022 consisted of the following (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended | ​ | Nine Months Ended
​ | ​ | September 30, | ​ | September 30,
​ |  | 2023 |  | 2022 |  | 2023 |  | 2022
Cost of sales | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Nonqualified stock options | ​ | $ | 367 | ​ | $ | 150 | ​ | $ | 1,240 | ​ | $ | 1,247
Research and development | ​ |  | ​ | ​ | ​ | ​ | ​ |  | ​ | ​ | ​ | ​
Nonqualified stock options | ​ | ​ | 488 | ​ |  | 476 | ​ | ​ | 1,329 | ​ |  | 1,412
Selling, general and administrative | ​ |  | ​ | ​ | ​ | ​ | ​ |  | ​ | ​ | ​ | ​
Nonqualified stock options | ​ | ​ | 2,083 | ​ |  | 2,166 | ​ | ​ | 5,304 | ​ |  | 5,297
Performance-based restricted stock units | ​ | ​ | 1,838 | ​ | ​ | 647 | ​ | ​ | 4,470 | ​ | ​ | 2,719
Restricted stock units | ​ | ​ | 430 | ​ | ​ | 454 | ​ | ​ | 1,341 | ​ | ​ | 1,382
Cash-settled performance-based share-based awards ("Liability Awards") | ​ | ​ | 591 | ​ | ​ | 705 | ​ | ​ | 1,662 | ​ | ​ | 1,634
Total selling, general and administrative | ​ | ​ | 4,942 | ​ | ​ | 3,972 | ​ | ​ | 12,777 | ​ | ​ | 11,032
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Stock-based compensation expense before taxes | ​ | $ | 5,797 | ​ | $ | 4,598 | ​ | $ | 15,346 | ​ | $ | 13,691
​ We recognize stock-based compensation expense (net of a forfeiture rate), for those awards which are expected to vest, on a straight-line basis over the requisite service period. We estimate the forfeiture rate based on our historical experience and expectations about future forfeitures. ​ Nonqualified Stock Options During the nine-month periods ended September 30, 2023 and 2022, we granted stock options representing 401,535 and 203,606 shares of our common stock, respectively. We use the Black-Scholes methodology to value the stock-based compensation expense for options. In applying the Black-Scholes methodology to the option grants, the fair value of our stock-based awards granted was estimated using the following assumptions for the periods indicated below: ​
​ | ​ | ​ | ​ | ​
​ | ​ | Nine Months Ended
​ | ​ | September 30,
​ | ​ | 2023 | ​ | 2022
Risk-free interest rate |  | 3.6% - 4.6% |  | 1.4% - 3.4%
Expected option term |  | 4.0 years |  | 4.0 years
Expected dividend yield |  | — |  | —
Expected price volatility |  | 44.6% - 47.1% |  | 46.2% - 47.4%
​ The average risk-free interest rate is determined using the U.S. Treasury rate in effect as of the date of grant, based on the expected term of the stock award. We determine the expected term of stock options using the historical exercise behavior of employees. The expected price volatility was determined using a weighted average of daily historical volatility of our stock price over the corresponding expected option term and implied volatility based on recent trends of the daily historical volatility. For awards with a vesting period, compensation expense is recognized on a straight-line basis over the service period, which corresponds to the vesting period. ​ As of September 30, 2023, the total remaining unrecognized compensation cost related to non-vested stock options was $22.5 million, which was expected to be recognized over a weighted average period of 2.3 years. ​ Stock-Settled Performance-Based Restricted Stock Units (“Performance Stock Units”) ​ During the nine-month periods ended September 30, 2023 and 2022, we granted performance stock units which represent up to 286,863 and 120,710 shares of our common stock, respectively. Conversion of the performance stock units occurs at the end of the relevant performance periods, or one year after the agreement date, whichever is later. The number of shares delivered upon vesting at the end of the performance periods are based upon performance against specified financial performance metrics and relative total shareholder return as compared to the Russell 2000 Index (“rTSR”), as defined in the award agreements. ​ We use Monte-Carlo simulations to estimate the grant-date fair value of the performance stock units linked to total shareholder return. The fair value of each performance stock unit was estimated as of the grant date using the following assumptions for awards granted in the periods indicated below: ​
​ | ​ | ​ | ​ | ​
​ | ​ | Nine Months Ended
​ | ​ | September 30,
​ | ​ | 2023 | ​ | 2022
Risk-free interest rate |  | 3.9% - 4.6% |  | 1.6% - 2.7%
Performance period |  | 2.8 years |  | 2.6 - 2.8 years
Expected dividend yield |  | — |  | —
Expected price volatility |  | 31.4% - 32.6% |  | 38.5% - 46.2%
​ The risk-free interest rate of return was determined using the U.S. Treasury rate at the time of grant with a term equal to the expected term of the award. The expected volatility was based on the weighted average volatility of our stock price and the average volatility of our compensation peer group's stock price. The expected dividend yield was assumed to be zero because, at the time of the grant, we had no plans to declare a dividend. Compensation expense is recognized using the grant-date fair value for the number of shares that are probable of being awarded based on the performance metrics. Each reporting period, this probability assessment is updated, and cumulative adjustments are recorded based on the financial performance metrics expected to be achieved. At the end of the performance period, cumulative expense is calculated based on the actual performance metrics achieved. As of September 30, 2023, the total remaining unrecognized compensation cost related to stock-settled performance stock units was $12.3 million, which is expected to be recognized over a weighted average period of 2.0 years. Liability Awards During the nine-month periods ended September 30, 2023 and 2022, we granted liability awards to our Chief Executive Officer with total target cash incentives in the amount of $1.3 million and $1.0 million, respectively. These awards entitle him to a target cash payment based upon our relative shareholder return as compared to the rTSR and achievement of specified performance metrics, as defined in the award agreements. During the nine-month period ended September 30, 2023, we granted additional performance stock units to certain employees that provide for settlement in cash upon our achievement of specified financial metrics. The cash payable upon vesting at the end of the service period is based upon performance against specified financial performance metrics and relative total shareholder return as compared to the rTSR, as defined in the award agreements. Compensation expense is recognized for the cash payment probable of being awarded based on the performance metrics. The fair value of these liability awards is measured at each reporting period until the awards are settled. These awards are classified as liabilities and reported in accrued expenses and other long-term obligations within our consolidated balance sheet. As of September 30, 2023, the total remaining unrecognized compensation cost related to cash-settled performance-based share-based awards was $3.6 million, which is expected to be recognized over a weighted average period of 2.0 years. ​ Restricted Stock Units During the nine-month periods ended September 30, 2023 and 2022, we granted restricted stock units to our non-employee directors representing 20,358 and 30,500 shares of our common stock, respectively. The expense recognized for restricted stock units is equal to the closing stock price on the date of grant, which is recognized over the vesting period. Restricted stock units granted to each director are subject to such director’s continued service through the vesting date, which is one year from the date of grant. As of September 30, 2023, the total remaining unrecognized compensation cost related to restricted stock units was $1.1 million, which will be recognized over the remaining vesting period.

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