12.Stock-Based Compensation

Our 2021 Equity Incentive Plan allows for the grant of, among other types of awards, stock options; restricted stock; RSUs; and PSUs.

Restricted Stock and Restricted Stock Units

RSU activity during the period was as follows:

​

​

The RSUs generally vest in the following installments: (i) one-third at the first anniversary of the award date, (ii) one-third at the second anniversary of the award date, and (iii) one-third at the third anniversary of the award date. As of March 31, 2024, approximately $26.9 million of compensation cost related to unvested shares of restricted stock and RSUs remained to be recognized. The cost is expected to be recognized over a weighted-average period of 1.5 years.

Performance-Based Restricted Stock Units

PSU activity during the period was as follows:

​

​

The PSUs have the following performance criteria:

The vesting and payout of the PSUs occur when the related service condition is completed, which is approximately three years after the grant date regardless of the duration of the stipulated performance period. The PSUs can be paid out in either Class A common stock or cash, at our election. Dividends accrue on PSUs and are paid upon vesting. As of March 31, 2024, approximately $2.3 million of compensation cost related to unvested PSUs remained to be recognized. The cost is expected to be recognized over a weighted-average period of 1.5 years.

The grant date fair value was determined using the Monte Carlo simulation method and is expensed ratably over the service period. Expected volatilities used in the fair value simulation were estimated using historical periods consistent with the remaining performance periods. The risk-free rate was based on the U.S. Treasury rate for a term commensurate with the expected life of the grant.