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Stock-Based Compensation
 3 Months Ended
Mar. 31, 2022
Share-based Payment Arrangement [Abstract]  
Stock-Based Compensation Stock-Based Compensation
Stock-based compensation expense includes grants of stock options, time-based restricted stock units ("Time-Based RSUs") and performance-based restricted stock units ("Performance-Based RSUs," together with Time-Based RSUs, the "RSUs"). In addition, stock-based payments to non-executive directors include grants of restricted stock awards ("RSAs"). Time-Based RSUs and RSAs are valued based on the intrinsic value of the difference between the exercise price, if any, of the award and the fair market value of WillScot Mobile Mini's Common Stock on the grant date. Performance-Based RSUs are valued based on a Monte Carlo simulation model to reflect the impact of the Performance-Based RSUs market condition. The probability of satisfying a market condition is considered in the estimation of the grant-date fair value for Performance-Based RSUs and the compensation cost is not reversed if the market condition is not achieved, provided the requisite service has been provided.
Restricted Stock Awards
The following table summarizes the Company's RSA activity:
20222021
Number of SharesWeighted-Average Grant Date Fair ValueNumber of SharesWeighted-Average Grant Date Fair Value
Balance December 31,36,176 $29.30 57,448 $11.75 
Balance March 31, 36,176 $29.30 57,448 $11.75 
Compensation expense for RSAs recognized in SG&A expense on the condensed consolidated statements of operations was $0.3 million and $0.2 million for the three months ended March 31, 2022 and 2021, respectively. At March 31, 2022, there was $0.2 million of unrecognized compensation cost related to RSAs that is expected to be recognized over the remaining weighted average vesting period of 0.2 years.
Time-Based RSUs
The following table summarizes the Company's Time-Based RSU activity:
20222021
Number of SharesWeighted-Average Grant Date Fair ValueNumber of SharesWeighted-Average Grant Date Fair Value
Balance December 31,997,451 $18.54 1,325,862 $13.46 
Granted357,639 $35.53 405,505 $27.20 
Forfeited(9,299)$27.20 (33,238)$13.62 
Vested(420,184)$16.21 (344,300)$13.31 
Balance March 31,925,607 $26.08 1,353,829 $17.61 
Compensation expense for Time-Based RSUs recognized in SG&A expense on the condensed consolidated statements of operations was $2.2 million and $2.1 million for the three months ended March 31, 2022 and 2021, respectively. At March 31, 2022, unrecognized compensation cost related to Time-Based RSUs totaled $22.1 million and is expected to be recognized over the remaining weighted average vesting period of 2.9 years.
Performance-Based RSUs
The following table summarizes the Company's Performance-Based RSU award activity:
20222021
Number of SharesWeighted-Average Grant Date Fair ValueNumber of SharesWeighted-Average Grant Date Fair Value
Balance December 31,1,536,394 $26.34 593,388 $14.88 
Granted745,079 $42.34 397,981 $39.10 
Forfeited(5,046)$39.10 (10,886)$14.70 
Vested(267,964)$13.22 — $— 
Balance March 31,2,008,463 $33.99 980,483 $24.70 
Compensation expense for Performance-Based RSUs recognized in SG&A expense on the condensed consolidated statements of operations was $3.8 million and $1.0 million for the three months ended March 31, 2022 and 2021, respectively. At March 31, 2022, unrecognized compensation cost related to Performance-Based RSUs totaled $55.1 million and is expected to be recognized over the remaining vesting period of 2.8 years.
Certain Performance-Based RSUs cliff vest based on achievement of the relative total stockholder return ("TSR") of the Company's Common Stock as compared to the TSR of the constituents in an index at the grant date over the performance period of three years. For 2022 grants, the TSR of the Company's Common Stock is compared to the TSR of the constituents in the S&P MidCap 400 Index. The target number of RSUs may be adjusted from 0% to 200% based on the TSR attainment levels defined by the Company's Compensation Committee. The 100% target payout is tied to performance at the 50% percentile, with a payout curve ranging from 0% (for performance less than the 25% percentile) to 200% (for performance above the 85% percentile).
Stock Options
The following tables summarize the Company's stock option activity:
WillScot OptionsWeighted-Average Exercise Price per ShareConverted
Mobile Mini Options		Weighted-Average Exercise Price per Share
Balance December 31 , 2021534,188 $13.60 1,527,643 $14.66 
Exercised— $— (227,258)$14.58 
Balance March 31, 2022534,188 $13.60 1,300,385 $14.67 
Fully vested and exercisable options, March 31, 2022534,188 $13.60 1,300,385 $14.67 
WillScot OptionsWeighted-Average Exercise Price per ShareConverted Mobile Mini OptionsWeighted-Average Exercise Price per Share
Balance December 31, 2020534,188 $13.60 2,031,455 $14.78 
Forfeited— $— (6,240)$12.19 
Exercised— $— (346,247)$15.89 
Balance March 31, 2021534,188 $13.60 1,678,968 $14.57 
Fully vested and exercisable options, March 31, 2021400,641 $13.60 1,678,968 $— 
WillScot Options
Compensation expense for stock option awards, recognized in SG&A expense on the condensed consolidated statements of operations, was $0.2 million for each of the three months ended March 31, 2022 and 2021.