We recognize compensation expense based on estimated grant date fair values for all share-based awards issued to employees and directors, including stock options, restricted stock units and performance share units. We estimate the fair value of share-based awards based on assumptions as of the grant date. We recognize compensation expenses for those awards expected to vest, on a straight-line basis over the requisite service period of the award, which is generally the vesting term of three years for restricted stock awards and stock options and the performance period for performance share units. For those awards in which there is no requisite service period, we immediately recognize the compensation expense. We recognize forfeitures as they occur as a reduction of compensation expense.

The total share-based compensation expense was $7.1 million, $8.1 million and $10.9 million for the years ended December 31, 2021, 2020 and 2019, respectively. As of December 31, 2021, $6.4 million of unrecognized compensation expense related to share-based compensation plans is expected to be recognized over a weighted-average period of 1.9 years.

We have one share-based compensation plan under which we may grant future awards, as described below, and one terminated share-based compensation plan under which awards remain outstanding.

The 2017 Performance Incentive Plan (“2017 PIP”) was approved by stockholders to provide incentives to our key employees. Awards under the 2017 PIP are generally not restricted to any specific form or structure and could include, without limitation, stock options, stock units, restricted stock awards, cash or stock bonuses and stock appreciation rights. There were 15.1 million shares of common stock reserved and authorized for issuance under the 2017 PIP. At December 31, 2021, there were 7.4 million shares of common stock authorized and available for grant under the 2017 PIP.

Under various incentive plans, we have granted certain employees non-qualified stock options, restricted stock units, performance share units and cash-settled stock units (“phantom stock units”). The Human Resources Committee of the Board of Directors has discretion to establish the terms and conditions for grants, including the number of shares, vesting and required service or other performance criteria. The maximum term of any award under the 2017 PIP and previous plans is ten years.

The exercise price of a stock option is equal to the closing price of our common stock on the option grant date and generally vest over four years. Options generally expire ten years from the date of grant or five years after the date of retirement, whichever is earlier.

The rights granted to the recipient of restricted stock unit awards generally accrue ratably over the restriction or vesting period, which is generally three years. We have also granted restricted stock unit awards which cliff vest three years from the grant date. Restricted stock unit awards are subject to forfeiture upon termination of employment prior to vesting, subject in some cases to early vesting upon specified events, including death or permanent disability of the grantee, termination of the grantee’s employment under certain circumstances or a change in control of the Company. We record compensation expense of restricted stock unit awards based on the fair value of the awards at the date of grant ratably over the period during which the restrictions lapse. Dividends are not paid on restricted stock units.

Awards that may be paid in cash are classified as liability awards due to their expected settlement in cash, and are included in Accrued liabilities and other in the Consolidated Balance Sheets. Compensation expense for these awards is measured based upon the fair value of the awards at the end of each reporting period. Awards payable only in shares are classified as equity awards due to their expected settlement in common stock. Compensation expense for these awards is measured based upon the fair value of the awards at the date of grant. Dividend equivalents are accrued for shares awarded to the Board of Directors and paid in the form of cash.

We have granted performance share unit awards to certain executive officers and senior management. Distributions under these awards are payable at the end of their respective performance periods in common stock or cash, at our discretion. The number of share units that vest can range from zero to 150% for the 2021, 2020 and 2019 awards, depending on achievement of a targeted performance metric for a performance period of three years inclusive of the year in which the award was granted. These awards are subject to forfeiture upon termination under certain circumstances prior to vesting. We expense the cost of the performance share unit awards based on the fair value of the awards at the date of grant and the estimated achievement of the performance metric, ratably over the performance period of three years.

In addition, we have granted phantom restricted and performance stock units to certain members of senior management. Phantom restricted stock units vest and are payable in three equal installments over a period of three years after the grant date. Phantom performance stock units vest at the end of their respective performance periods. The number of phantom performance share units that vest can range from zero to 150% for the 2021, 2020 and 2019 awards, depending on achievement of a targeted performance metric for a performance period of three years inclusive of the year in which the award was granted. Phantom stock units are not shares of our common stock and therefore the recipients of these awards do not receive ownership interest in the Company or stockholder voting rights. These awards are subject to forfeiture upon termination of employment prior to vesting, subject in some cases to early vesting upon specified events, including death or permanent disability of the grantee, termination of the grantee’s employment under certain circumstances or a change in control of the Company. All phantom stock unit awards are classified as liability awards due to their expected settlement in cash, and are included in Accrued liabilities and other in the Consolidated Balance Sheets. Compensation expense for these awards is measured based upon the fair value of the awards at the end of each reporting period and, for Phantom performance shares, the estimated achievement of the performance metric. Dividends are not paid on phantom stock units. Total compensation expense for phantom awards was $33.3 million, $3.1 million and $3.0 million for the years ended December 31, 2021, 2020 and 2019, respectively.

In connection with the Merger, certain executive officers of the Company may become entitled to payments and benefits that may be treated as excess parachute payments within the meaning of 280G of the Internal Revenue Code of 1986, as amended ( “Section 280G” and the “Code”, respectively ). To mitigate the potential impact of Section 280G and Section 4999 of the Code on the Company and its applicable executive officers, effective December 17, 2021 the Company’s Board of Directors approved the acceleration into December 2021 of the vesting and payment of certain restricted stock units, performance share units, phantom stock units and cash awards that otherwise would have been payable to the executive officers on or prior to the closing of the Merger. These actions were intended to preserve compensation-related corporate income tax deductions for the Company that might otherwise be disallowed through the operation of Section 280G and to mitigate or eliminate the amount of excise tax that may be payable by the executive pursuant to Section 4999 of the Code in connection with Section 280G in certain circumstances.

There were no options granted during the years ended December 31, 2021, 2020 and 2019.

Stock option awards as of December 31, 2021 and 2020, and changes during the year ended December 31, 2021 were as follows:

	Shares Under Option (thousands)			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)		Aggregate Intrinsic Value (millions)
Outstanding at December 31, 2020		276			23.59		0.9	$	—
Cancelled/forfeited/expired		(191	)		24.53
Outstanding at December 31, 2021		85			21.47		0.2		—
Vested and exercisable at December 31, 2021		85		$	21.47		0.2	$	—

There was no unrecognized compensation expense related to stock options as of December 31, 2021.

Nonvested restricted stock unit awards as of December 31, 2021 and 2020, and changes during the year ended December 31, 2021 were as follows:

	Shares (thousands)			Weighted Average Grant Date Fair Value
Nonvested at December 31, 2020		1,440		$	3.18
Granted		1,042			4.73
Vested		(2,370	)		3.84
Forfeited		(112	)		3.67
Nonvested at December 31, 2021		—		$	—

Included above are 1,486 shares of restricted stock units that were accelerated vested into December 2021. As of December 31, 2021, there was $3.2 million in unrecognized share-based compensation related to restricted stock units, which is expected to be recognized over a weighted-average period of 1.9 years. The fair value of these awards was determined based on our stock price on the grant date reduced by the present value of expected dividends through the vesting period.

Nonvested performance share unit awards as of December 31, 2021 and 2020, and changes during the year ended December 31, 2021, were as follows:

	Shares (thousands)			Weighted Average Grant Date Fair Value
Nonvested at December 31, 2020		2,076		$	3.94
Granted		850			4.52
Performance adjustment		649			3.11
Vested		(3,291	)		3.99
Forfeited		(107	)		3.61
Nonvested at December 31, 2021		177		$	2.93

Included above are 1,726 of performance share units that were accelerated vested into December 2021. As of December 31, 2021, there was $3.2 million of unrecognized compensation expense related to performance share unit awards, which is expected to be recognized over a weighted-average period of 1.8 years.

Phantom stock unit awards as of December 31, 2021 and 2020, and changes during the year ended December 31, 2021, were as follows:

	Shares (thousands)			Weighted Average Grant Date Fair Value
Nonvested at December 31, 2020		4,603		$	3.25
Granted		1,719			3.26
Performance adjustment		208			3.54
Vested		(3,465	)		3.27
Forfeited		(708	)		3.22
Nonvested at December 31, 2021		2,357		$	3.26

Included above are 1,762 shares of phantom stock units that were accelerated vested into December 2021. As of December 31, 2021, there was $24.0 million of unrecognized compensation expense related to phantom stock unit awards based on the price of our common stock on that date, which is expected to be recognized over a weighted-average period of 1.5 years.