Stock-Based Compensation

9 Months Ended

Sep. 30, 2021

Share-based Payment Arrangement [Abstract]

Pursuant to the Company's Omnibus Incentive and Equity Plan (the "Plan"), officers, employees and directors may be granted equity-based awards, including restricted stock units ("RSUs"), performance stock units ("PSUs"), stock options and unrestricted shares of common stock. At September 30, 2021, 805,875 shares of common stock remained available for issuance of the 3,370,000 shares that are authorized for issuance under the Plan.

Stock-based compensation expense is summarized as follows:


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
(in thousands)
Stock-based compensation expense	$	5,989	$	6,299	$	21,833	$	16,412

Restricted Stock Units

Each RSU entitles the holder to one share of common stock when the restriction expires. RSUs may be time-vested or performance-contingent (PSUs) that convert into RSUs after performance measurement is complete and generally vest in one to three years. Shares that are issued upon vesting are newly issued shares from the Plan and are not issued from treasury stock.

RSU activity, inclusive of PSUs, for the nine months ended September 30, 2021 is summarized as follows:


	Number of Shares	Weighted Average Grant Date Fair Value
Outstanding at December 31, 2020	533,185	$	106.19
Granted	107,216	$	268.56
Forfeited	(22,752)	$	127.96
Settled	(184,589)	$	123.17
Outstanding at September 30, 2021	433,060	$	138.00

For the nine months ended September 30, 2021 and 2020, a total of 72,795 and 63,566 RSUs, respectively, were withheld by the Company as a result of net share settlements to settle minimum employee tax withholding obligations. The Company paid $19.4 million and $5.6 million for the nine months ended September 30, 2021 and 2020, respectively, in minimum employee tax withholding obligations related to RSUs withheld for the net share settlements.

During the nine months ended September 30, 2021, the Company granted 26,425 PSUs that contain performance-based metrics in addition to a service condition. Compensation expense for PSUs is generally recognized over a three-year service period based upon the value determined using a combination of (i) the intrinsic value method, for awards that contain a performance metric that represents a "performance condition" in accordance with ASC 718, and (ii) the Monte Carlo simulation valuation model for awards that contain a "market condition" performance metric under ASC 718. Compensation expense for PSU awards that contain a market condition is fixed at the date of grant and will not be adjusted in future periods based upon the achievement of the market condition. Compensation expense for PSU awards with a performance condition is recorded each period based upon a probability assessment of the expected outcome of the performance metric with a final adjustment upon measurement at the end of the performance period.

As of September 30, 2021, unamortized stock-based compensation expense for unvested RSUs and PSUs was $30.8 million, with a weighted-average remaining contractual life of 1.2 years.