The Company believes that stock-based compensation aligns the interests of its employees and directors with those of its stockholders. Stock-based compensation provides incentives to retain and motivate executive officers and key employees responsible for driving Company performance and maintaining important relationships that contribute to the growth of the Company. As of December 31, 2020, the Company has two stock-based compensation plans:

2010 Plan — On June 13, 2019, the stockholders of the Company approved the Second Amended and Restated 2010 Incentive Award Plan (the "2010 Plan"). This plan permits the grant of incentive stock options, nonqualified stock options, dividend equivalent rights, stock payments, deferred stock, restricted stock awards, RSUs, performance shares and other incentive awards, stock appreciation rights and cash awards to its employees, directors and consultants. As of December 31, 2020, there were 2.6 million shares available for issuance under the 2010 Plan.

2012 Plan — The Board of Directors adopted the Second Amended and Restated 2012 Employment Inducement Incentive Award Plan on April 26, 2018 (the "2012 Plan"). This plan allows for grants of stock to employees as employment inducement awards pursuant to NYSE rules. The terms of the 2012 Plan are similar to the 2010 Plan. As of December 31, 2020, there were 0.1 million shares available for issuance under the 2012 Plan.

Restricted Stock Units — The Company issues RSUs with (i) service conditions, (ii) performance conditions, (iii) a combination of performance and service conditions and (iv) a combination of market and service conditions. RSUs generally vest over one- to five-year periods, and the RSUs that have performance conditions are based on the achievement of specified annual financial or other targets. Beginning in 2020, the Company also included market conditions based on relative total shareholder return ("TSR") or "TSR Awards." These TSR Awards vest solely based on achievement of TSR relative to an objectively selected group of industry peers over a three-year period, with payouts ranging from zero to 200% of the target award.

The fair value of each RSU is based on the grant-date fair market value of the awards. The fair value of the Company's RSUs were determined on the grant date based on the closing market price for the Company's stock. The fair value for the TSR Awards was $49.11 per share, which was determined on the grant date using a Monte Carlo simulation model based on the following assumptions:

Compensation expense for RSUs is determined based on the grant-date fair value of those awards, net of an estimated forfeiture rate. The forfeiture rate estimates the number of awards that will eventually vest and is based on historical vesting patterns. Compensation expense for RSUs with performance conditions based on financial targets are measured on the amount of shares ultimately expected to vest, estimated at each reporting date based on management’s expectations regarding the relevant performance criteria. Compensation expense for all other RSUs are recognized on a straight-line basis, net of an estimated forfeiture rate over the requisite service period of the award.

A summary of the status of the Company’s unvested RSUs as of December 31, 2020 and changes during the year then ended are presented below (number of units in millions, except fair value per unit):


	Service Conditions	Performance/Market and Service Conditions	Total	Weighted-Average Grant-Date Fair Value Per Unit
Unvested RSUs outstanding at December 31, 2019	0.5	0.7	1.2		$	63.21
Granted	0.2	0.3	0.5		$	61.23
Vested	(0.1)	(0.4)	(0.5)		$	59.51
Unvested RSUs outstanding at December 31, 2020	0.6	0.6	1.2		$	63.66
Unvested and expected to vest RSUs outstanding at December 31, 2020	0.5	0.6	1.1		$	63.51

As of December 31, 2020, there was unrecognized compensation expense of $37.9 million related to unvested RSUs based on awards that are expected to vest. The unrecognized compensation expense is expected to be recognized over a weighted-average period of approximately 1.9 years. The fair value of RSUs that vested during 2020, 2019 and 2018 was $34.4 million, $38.7 million and $16.4 million, respectively. The weighted-average grant-date fair value of RSUs granted during 2020, 2019 and 2018 was $61.23, $62.26 and $74.61 per unit, respectively. The total number of shares vested in the table above includes 0.2 million shares surrendered by the employees to the Company for payment of employees' income taxes. The surrendered shares are available for issuance under the 2010 Plan.

Employee Stock Purchase Plan — The stockholders of the Company approved the Second Amended and Restated 2010 Employee Stock Purchase Plan (“ESPP”) on June 18, 2020. The ESPP allows eligible employees to purchase common stock of the Company, through payroll deductions, at a 15 percent discount of the lower of the market price on the first day or the last day of the semi-annual purchase periods. Participants are required to hold the shares for a 12-month period after the purchase date. The ESPP is intended to qualify as an employee stock purchase plan under the Internal Revenue Service ("IRS") Code Section 423. Eligible employees may contribute up to a certain percentage set by the plan administrator of their eligible earnings toward the purchase of the stock (subject to certain IRS limitations). As of December 31, 2020, there were 1.3 million shares available for issuance under the ESPP.

Shares of common stock are transferred to participating employees at the conclusion of each six-month offering period, which ends on the last business day of the month in March and September each year. Compensation expense is measured using a Black-Scholes valuation model. The fair values of the options granted under the ESPP were estimated using the Black-Scholes valuation model at the date of grant based on the following assumptions during the years ended December 31, 2020, 2019 and 2018:



	2020		2019		2018
Expected term (years)	0.5		0.5		0.5
Dividend yield	—		—		—
Expected volatility	32.0 - 63.3%		25.0 - 38.5%		23.0 - 30.3%
Weighted-average risk-free interest rate	0.1 - 1.8%		2.1 - 2.4%		1.2 - 1.9%
Average Black-Scholes valuation per share	$	12.53	$	17.11	$	15.09

Shares issued (millions)	0.4		0.2		0.2
Stock-based compensation expense (millions)	$	4.0	$	4.1	$	2.7

Stock Options and Liability Awards — The Company has not granted stock options since 2012 and liability awards since 2019. The activity related to exercised stock options and liability awards during the years ended 2020, 2019 and 2018 was insignificant. There were no liability awards outstanding at December 31, 2020, and the number of outstanding liability awards at the end of 2019 and 2018 was insignificant. The number of outstanding stock options at the end of these years were also insignificant.

Deferred Compensation Plan — The Company’s Deferred Compensation Plan, which became effective on June 1, 2017 and has been amended from time to time (the "DCP"), allows for eligible management and highly compensated key employees to elect to defer a portion of their compensation to later years. These deferrals are immediately vested and are subject to investment risk and a risk of forfeiture under certain circumstances. Participants may choose from various investment options representing a broad range of asset classes. The Company’s deferred compensation plan liability was $14.4 million and $11.8 million at December 31, 2020 and 2019, respectively, which was primarily included in other long-term liabilities. The Company established a rabbi trust to fund the deferred compensation plan (see Note 15. Fair Value Measurements).