The Company has stock-based compensation plans that provide for the award of equity incentives, including stock options, stock awards, RSUs, PSUs, and purchase rights. As of March 31, 2022, the Company had stock options outstanding under the 2018 Plan and the 2006 Equity Incentive Plan (the “2006 Plan”), RSUs and PSUs outstanding under the 2018 Plan, and purchase rights issued under the ESPP. The 2018 Plan became effective in connection with the Company’s initial public offering at which time the 2006 Plan was terminated. Outstanding awards under the 2006 Plan continue to be subject to the terms and conditions of the 2006 Plan.

As of March 31, 2022, 4,500,890 shares were subject to outstanding awards under the 2018 plan and 14,372,738 shares were available for issuance under the 2018 Plan. As of March 31, 2022, 1,093,901 shares were subject to outstanding stock options under the 2006 Plan.

The Company recognized total stock-based compensation cost related to equity incentive awards as follows (in thousands):

	For the Three Months Ended March 31,
	2022		2021
Stock-based compensation cost:
Stock options	$	2,161	$	3,223
Restricted stock units		22,128		11,769
Performance share units		7,813		2,832
Employee stock purchase plan		1,269		1,303
Total stock-based compensation cost	$	33,371	$	19,127

The amount of stock-based compensation cost above capitalized to internally developed software and cloud computing arrangements in accordance with the accounting guidance for internal-use software was $0.6 million and $0.2 million for the three months ended March 31, 2022 and 2021, respectively. The Company uses the straight-line attribution method for recognizing stock-based compensation expense.

The following table summarizes stock option activity for the Company’s stock-based compensation plans for the three months ended March 31, 2022:

						Weighted
				Weighted		Average		Aggregate
				Average		Remaining		Intrinsic
				Exercise		Contractual		Value
	Shares			Price		Life (in Years)		(in thousands)
Options outstanding as of January 1, 2022		2,278,789		$	33.52		6.30	$	217,824
Options granted		—		$	—
Options exercised		(107,623	)	$	21.20
Options cancelled or expired		(4,291	)	$	111.83
Options outstanding as of March 31, 2022		2,166,875		$	33.98		6.08	$	141,996
Options exercisable as of March 31, 2022		1,778,867		$	28.90		5.81	$	125,598

The total intrinsic value of options exercised during the three months ended March 31, 2022 and 2021 was $7.7 million and $41.9 million, respectively. As of March 31, 2022, $8.9 million of total unrecognized compensation cost related to stock options was expected to be recognized over a weighted average period of approximately 1.5 years.

The following table summarizes RSU activity for the Company’s stock-based compensation plans for the three months ended March 31, 2022:

				Weighted
				Average
				Grant Date Fair
	Restricted Stock Units			Value Per Share
RSUs outstanding as of January 1, 2022		2,861,963		$	99.41
RSUs granted		1,903,805			97.15
RSUs vested		(466,497	)		80.86
RSUs cancelled		(98,343	)		94.81
RSUs outstanding as of March 31, 2022		4,200,928		$	100.56

Stock-based compensation cost for RSUs is recognized on a straight-line basis in the consolidated statements of operations over the period during which the participant is required to perform services in exchange for the award, based on the fair value of the Company’s underlying common stock on the grant date. The vesting period of each RSU grant is generally three or four years for employees and one year for non-employee directors.

As of March 31, 2022, $374.0 million of total unrecognized compensation cost related to RSUs was expected to be recognized over a weighted average period of approximately 2.8 years.

The following table summarizes PSU activity for the Company’s stock-based compensation plans for the three months ended March 31, 2022:

				Weighted
				Average
				Grant Date Fair
	Performance Share Units			Value Per Share
PSUs outstanding as of January 1, 2022		114,890		$	147.26
PSUs granted		223,363			98.90
PSUs vested		(38,291	)		147.26
PSUs cancelled		—			—
PSUs outstanding as of March 31, 2022		299,962		$	111.25

Stock-based compensation cost for PSUs is recognized on a graded vesting basis in the consolidated statements of operations over the period during which the participant is required to perform services in exchange for the award, based on the fair value of the Company’s underlying common stock on the grant date and the number of PSUs expected to be earned over the service period. Each PSU grant will vest in annual tranches over a three-year service period. Total PSUs earned for grants made in 2022 and 2021 may vary between 0% and 250% of the PSUs granted based on the attainment of company-specific performance targets during the related three-year period and continued service. The performance targets are established by the Compensation and Leadership Development Committee of the Board of Directors. All of the PSUs earned will be settled through the issuance of an equivalent number of shares of the Company’s common stock based on the payout level achieved.

As of March 31, 2022, $53.3 million of total unrecognized compensation cost related to PSUs was expected to be recognized over a weighted average period of approximately 2.0 years.

During the three months ended March 31, 2022, 85,926 shares of common stock were purchased under the ESPP.

As of March 31, 2022, there was approximately $1.7 million of unrecognized stock-based compensation cost related to the ESPP that is expected to be recognized over the remaining term of the offering period that began on February 1, 2022 and will end on July 31, 2022.

For the periods presented, the fair value of ESPP purchase rights was estimated using the Black-Scholes option-pricing model with the following assumptions:

	For the Three Months Ended March 31,
	2022		2021
Fair market value of common stock	$	111.00	$	152.35
Volatility	33%		34%
Expected term	0.5 years		0.5 years
Expected dividend yield	n/a		n/a
Risk-free interest rate	0.48%		0.08%