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. In March 2021, 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. The Company's stockholders approved the Plan amendment in May 2021. Forfeited awards under the Plan generally become available for reissuance. As of December 31, 2023, 832,273 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. The Company’s vested stock grants to the Board and related expense for the years ended December 31, 2023, 2022 and 2021, was as follows:


	Year Ended December 31,
(millions, except stock grants)	2023		2022		2021
Vested stock grants		18,660		14,635		13,420
Stock-based compensation expense	$	0.6	$	0.4	$	0.5

During the year ended December 31, 2023, the Company granted 126,931 restricted stock units to certain executives that vest over three years.

During the year ended December 31, 2022, the Company granted 1,057 and 187,574 restricted stock units to certain executives and employees at a weighted average grant-date fair value of $31.82 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 at a weighted average grant-date fair value of $34.45 that vested over two and three years, respectively.

On October 23, 2023, the Board approved a change in the vesting date for the RSU’s granted in 2021 that were originally expected to vest as of December 31, 2023, to December 1, 2023. The change in the vesting date was considered a Type 1 modification, as the acceleration of the vesting date did not change the expectation that the awards would ultimately vest, and accordingly, the remaining compensation expense related to the awards was recognized and did not result in any additional compensation expense.

Nonvested RSU's as of December 31, 2023, and changes during the year ended December 31, 2023 were as follows:


	Shares			Weighted Average Grant-Date Fair Value
Nonvested as of December 31, 2022		338,448		$	33.28
Granted		126,931			34.57
Vested		(167,620	)		35.00
Forfeited		(6,972	)		33.83
Nonvested as of December 31, 2023		290,787		$	32.89

During the years ended December 31, 2022 and 2021, 188,887 and 5,615 RSU's, respectively, vested at a weighted average grant-date fair value of $33.88 and $26.71, respectively.

The Company's stock-based compensation expense related to RSU's during the years ended December 31, 2023, 2022 and 2021, which was included in General and administrative expenses within the Consolidated Statements of Income, was as follows:


	Year Ended December 31,
(millions)	2023		2022		2021
Stock-based compensation expense	$	5.1	$	5.7	$	4.6

Unrecognized stock-based compensation expense related to RSU's and the respective weighted average periods in which the expense will be recognized as of December 31, 2023 was as follows:


	Year Ended December 31,
(millions)	2023
Unrecognized stock-based compensation	$	5.4
Weighted average (years)		1.7

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 executives' 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, 2023, 2022 and 2021 of 126,921, 132,304 and 50,868, respectively, under the Performance-Based Incentive Program at a weighted average grant-date fair value of $34.57, $30.80 and $34.97, respectively. The performance target for the PSU awards 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 PSU awards granted in 2023 ("2023 PSU's") and 2022 ("2022 PSU's") are 253,842 and 258,114, respectively. The Company is currently recognizing expense for the 2023 PSU's and 2022 PSU's based on an expected payout of 129,459 shares and 196,167 shares, respectively.

On October 23, 2023, the Board approved a change in the vesting date for the PSU’s granted in 2021 (“2021 PSU’s”) that were originally expected to vest as of December 31, 2023, to December 1, 2023. In addition, the Board approved the maximum payout for the awards. The Company had been recognizing expense based on the maximum payout before the modification. The changes to the 2021 PSU’s noted above were considered a Type 1 modification, as the fair value of the awards before and after the modification were the same, and accordingly, the remaining compensation expense related to the 2021 PSU’s was recognized and did not result in any additional compensation expense.

Nonvested PSU’s as of December 31, 2023, and changes during the year ended December 31, 2023 were as follows:


	Shares			Weighted Average Grant-Date Fair Value
Nonvested as of December 31, 2022		177,605		$	31.94
Granted		126,921			34.57
2021 PSU's(1)		47,730			34.97
Vested		(95,460	)		34.97
Forfeited		(5,334	)		33.48
Nonvested as of December 31, 2023		251,462		$	32.66

(1)

During the year ended December 31, 2023, the Company issued an additional 47,730 shares due to the maximum performance targets being achieved for the 2021 PSU’s. As noted above, the 2021 PSU’s vested on December 1, 2023.

During the years ended December 31, 2022 and 2021, 80,979 and 112,328 PSU’s, respectively, expired at a weighted average grant-date fair value of $37.89 and $33.28, respectively, due to the performance targets not being met.

The Company's stock-based compensation expense related to PSU’s during the years ended December 31, 2023, 2022 and 2021, which was included in General and administrative expenses within the Consolidated Statements of Income, was as follows:


	Year Ended December 31,
(millions)	2023		2022		2021
Stock-based compensation expense	$	4.7	$	2.9	$	1.0

Unrecognized stock-based compensation expense related to PSU’s based on current projections and the respective weighted average periods in which the expense will be recognized as of December 31, 2023 was as follows:


	Year Ended December 31,
(millions)	2023
Unrecognized stock-based compensation	$	5.1
Weighted average (years)		1.6

The Company could recognize additional future stock-based compensation of $4.2 million and $1.9 million for the 2023 PSU's and 2022 PSU's, respectively, if the maximum performance targets are achieved.