The Company measures stock-based compensation expense at the grant date, based on the estimated fair value of the award based on assumptions, primarily the stock price, as of the grant date. The expense is recognized on a straight-line basis over the requisite employee service period or performance period (generally the vesting period) for awards expected to vest. For stock grants in which there is no requisite service period, the Company immediately recognizes the compensation expense. If an award is later modified, the Company may measure the award based on the estimated fair value at the modification date and recognize expense over the remaining requisite employee service period or performance period. The Company accounts for forfeitures of stock-based awards as they occur.

The Company has an amended and restated long-term incentive plan (the "Plan") under which the Company may grant future awards. On March 4, 2021, the Company’s Board of Directors (the “Board”) approved an amendment to the Plan that increased the number of shares of common stock available under the Plan from 3,775,000 to 4,775,000. Company stockholders approved the Plan amendment on May 12, 2021. Forfeited awards under the Plan generally become available for reissuance. At December 31, 2022, 1,054,163 shares remained available for grant under the Plan.

Stock-based compensation expense related to vested stock grants were included in General and administrative expenses within the Consolidated Statements of Income (Loss). The Company’s vested stock grants to the Board and related expense for the years ended December 31, 2022, 2021 and 2020, was as follows:


	Year Ended December 31,
(millions, except stock grants)	2022		2021		2020
Vested stock grants		14,635		13,420		25,066
Stock-based compensation expense	$	0.4	$	0.5	$	0.5

During the year ended December 31, 2022, the Company granted 1,057 and 187,574 restricted stock units to certain executives and employees that vest over one and three years, respectively.

During the year ended December 31, 2021, the Company granted 160,843 and 152,659 restricted stock units to certain executives and employees that vest over two and three years, respectively.

Nonvested restricted stock units as of December 31, 2022, and changes during the year ended December 31, 2022 were as follows:


	Shares			Weighted Average Grant-Date Fair Value
Nonvested as of December 31, 2021		355,434		$	34.40
Granted		188,631			31.82
Vested		(188,887	)		33.88
Forfeited		(16,730	)		33.76
Nonvested as of December 31, 2022		338,448		$	33.28

The Company's stock-based compensation expense related to restricted stock units during the years ended December 31, 2022, 2021 and 2020, which was included in General and administrative expenses within the Consolidated Statements of Income (Loss), was as follows:


	Year Ended December 31,
(millions)	2022		2021		2020
Stock-based compensation expense	$	5.7	$	4.6	$	1.1

Unrecognized stock-based compensation expense related to restricted stock units and the respective weighted average periods in which the expense will be recognized as of December 31, 2022 was as follows:


	Year Ended December 31,
(millions)	2022
Unrecognized stock-based compensation	$	6.4
Weighted average (years)		1.9

In September 2014, the Board authorized a performance-based incentive program under the Plan (“Performance-based Incentive Program”), whereby the Company may issue PSU’s to certain individuals that represent shares potentially issuable in the future. The objective of the Performance-Based Incentive Program is to link compensation to business performance, encourage the ownership of the Company’s common stock, retain key employees and reward certain employees' performance. The Performance-Based Incentive Program provides participants with the opportunity to earn vested common stock if certain performance targets are achieved over the cumulative three-year period starting in the year of grant and the participants satisfy service-based vesting requirements. The stock-based compensation expense associated with PSU’s is recognized on a straight-line basis over the shorter of the vesting period or minimum service period and dependent upon the probable outcome of the number of shares that will ultimately be issued based on the achievement of the performance target defined in the award over the cumulative three-year period.

The Company granted awards during the years ended December 31, 2022, 2021 and 2020 of 132,304, 50,868 and 96,056, respectively, under the Performance-Based Incentive Program. The performance target for the PSU awards granted in 2022 ("2022 PSU's") and 2021 (“2021 PSU’s”) is based on the achievement of a certain level of operating income, excluding depreciation and amortization, subject to certain discretionary adjustments by the Board, over three-year performance periods. The ultimate number of shares issued could change depending on the Company's results over the performance period. The maximum amount of shares that could be issued for the 2022 PSU's and 2021 PSU's are 258,114 and 97,096, respectively. The Company is currently recognizing expense for the 2022 PSU's based on a payout of 187,133 shares and the maximum payout of 97,096 shares for the 2021 PSU's.

The performance target for the awards granted in 2020 (“2020 PSU’s”) was based on the achievement of free cash flow before cash taxes and interest payments over the cumulative three-year period starting in the year of grant, subject to certain discretionary adjustments by the Board. The 2020 PSU’s expired December 31, 2022 and no awards vested, as the target for these awards was not met primarily due to the impact of COVID-19 during the year ended December 31, 2020. Therefore, no compensation expense was recognized for the 2020 PSU’s during the years ended December 31, 2022, 2021 and 2020. Additionally, during the year ended December 31, 2020, the Company reversed $1.4 million of compensation expense related to the PSU awards granted in 2019 ("2019 PSU's"), which had previously been recognized during the year ended December 31, 2019. Due to the impact of COVID-19 during the year ended December 31, 2020, the Company determined the performance target was not expected to be met. The 2019 PSU's expired December 31, 2021, and no awards vested, as the target was not met.

Nonvested PSU’s as of December 31, 2022, and changes during the year ended December 31, 2022 were as follows:


	Shares			Weighted Average Grant-Date Fair Value
Nonvested as of December 31, 2021		135,383		$	36.79
Granted		132,304			30.80
Forfeited		(9,103	)		34.62
Expired		(80,979	)		37.89
Nonvested as of December 31, 2022		177,605		$	31.94

The Company's stock-based compensation expense (net reduction of expense) related to PSU’s during the years ended December 31, 2022, 2021 and 2020, which was included in General and administrative expenses within the Consolidated Statements of Income (Loss), was as follows:


	Year Ended December 31,
(millions)	2022		2021		2020
Stock-based compensation expense	$	2.9	$	1.0	$	(1.0	)

Unrecognized stock-based compensation expense related to PSU’s and the respective weighted average periods in which the expense will be recognized as of December 31, 2022 was as follows:


	Year Ended December 31,
(millions)	2022
Unrecognized stock-based compensation	$	5.3
Weighted average (years)		1.8

The Company could recognize additional future stock-based compensation of $2.2 million for the 2022 PSU's if the maximum performance target is achieved.