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 $1.6 million, $0.4 million, and $0.1 million for the years ended December 31, 2021, 2020, and 2019 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 2021:

						Weighted
				Weighted		Average		Aggregate
				Average		Remaining		Intrinsic
				Exercise		Contractual		Value
	Shares			Price		Life (in Years)		(in thousands)
Options outstanding as of January 1, 2021		3,537,014		$	28.17		6.89	$	483,577
Options granted		—		$	—
Options exercised		(1,223,395	)	$	17.80
Options cancelled or expired		(34,830	)	$	42.24
Options outstanding as of December 31, 2021		2,278,789		$	33.52		6.30	$	217,824
Options exercisable as of December 31, 2021		1,753,067		$	27.80		5.97	$	177,609

A summary of options outstanding and vested as of December 31, 2021, is as follows:

	Options Outstanding				Options Vested and Exercisable
Exercise	Number		Weighted Average		Number Vested		Weighted Average
Prices	Outstanding		Life (in Years)		and Exercisable		Life (in Years)
$1.90 to $6.40		18,612		1.1		18,612		1.1
$8.04 to $11.72		72,717		2.3		72,717		2.3
$12.20 to $15.06		380,304		4.7		380,304		4.7
$16.06 to $24.00		706,791		6.1		639,948		6.1
$31.99 to $42.21		655,237		7.0		441,953		7.0
$55.10 to $99.65		410,736		7.8		185,749		7.8
$124.35 to $151.76		34,392		7.8		13,784		6.5
		2,278,789				1,753,067

The total intrinsic value of options exercised during 2021, 2020, and 2019 was $173.2 million, $243.8 million, and $318.5 million, respectively.

There were no options granted during the year ended December 31, 2021. The weighted average grant date fair value of options granted during the years ended December 31, 2020, and 2019 was $34.00 and $19.80 per share, respectively. During the year ended December 31, 2021, 1,051,226 options vested. There were 525,722 options unvested as of December 31, 2021.

As of December 31, 2021, $11.1 million of total unrecognized compensation cost related to stock options was expected to be recognized over a weighted average period of approximately 1.6 years.

All options granted to participants, including employees and non-employee directors, are measured based on the grant date fair value of the awards and recognized in the consolidated statements of operations over the period during which the participant is required to perform services in exchange for the award. The vesting period is generally four years for employees and one year for non-employee directors. For the options granted during the periods presented, the fair value was estimated using the Black-Scholes option-pricing model with the following assumptions:

	For the Year Ended December 31,
	2021		2020	2019
Fair market value of common stock		—	$66.26 - $151.76	$39.76 - $79.25
Volatility		—	43% - 44%	40%
Expected term		—	5-6 years	5-6 years
Expected dividend yield		—	n/a	n/a
Risk-free interest rate		—	0.33% - 1.25%	1.68% - 2.65%

Options are granted with an exercise price per share not less than the per share fair market value of the Company’s common stock on the grant date. The fair market value per share of the Company’s common stock for purposes of determining stock-based compensation is the closing price of the Company’s common stock as reported on the applicable grant date.

Expected volatility for stock options is based on a combination of annualized daily historical volatility of the Company’s stock price and the historical and implied volatility of comparable publicly traded companies over a similar expected term.

The expected term represents the period that the Company’s stock-based awards are expected to be outstanding. Given the Company’s relative inexperience of significant exercise activity, the expected term assumptions were determined based on application of the simplified method of expected term calculation by averaging the contractual life of option grants and the vesting period of such grants. This application, when coupled with the contractual life of ten years and average vesting term of four years for employees and one year for non-employee directors, creates an expected term of six years and five years, respectively.

The risk-free interest rate was based on the rate for a U.S. Treasury zero-coupon issue with a term that closely approximates the expected life of the option grant at the date nearest the option grant date.

The following table summarizes RSU activity for the Company’s stock-based compensation plans for 2021:

				Weighted
				Average
				Grant Date Fair
	Restricted Stock Units			Value Per Share
RSUs outstanding as of January 1, 2021		2,359,063		$	70.17
RSUs granted		1,282,177		$	138.79
RSUs vested		(524,246	)	$	68.36
RSUs cancelled		(255,031	)	$	90.66
RSUs outstanding as of December 31, 2021		2,861,963		$	99.41

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 four years for employees and one year for non-employee directors. As of December 31, 2021, $220.5 million of total unrecognized compensation cost related to RSUs was expected to be recognized over a weighted average period of approximately 2.9 years.

The following table summarizes PSU activity for the Company’s stock-based compensation plans for 2021:

				Weighted
				Average
	Performance Share			Grant Date Fair
	Units			Value Per Share
PSUs outstanding as of January 1, 2021		—		$	—
PSUs granted		120,632		$	147.39
PSUs vested		—		$	—
PSUs cancelled		(5,742	)	$	150.00
PSUs outstanding as of December 31, 2021		114,890		$	147.26

During the first quarter of 2021, PSUs were granted for the first time. 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 units earned for grants made in 2021 may vary between 0% and 250% of the units 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 December 31, 2021, $19.9 million of total unrecognized compensation cost related to PSUs was expected to be recognized over a weighted average period of approximately 1.9 years.

The ESPP is intended to qualify as an employee stock purchase plan under Section 423 of the Internal Revenue Code of 1986, as amended. Under the ESPP, employee purchases occur at the end of discrete offering periods. Offering periods begin on August 1 and February 1 each year (or such other date determined by our Board of Directors or our Compensation and Leadership Development Committee).

Under the ESPP, eligible employees can acquire shares of the Company’s common stock by accumulating funds through payroll deductions. Employees generally are eligible to participate in the ESPP if they are a U.S. employee and are employed for at least 20 hours per week. The Company may impose additional restrictions on eligibility. Eligible employees can select a rate of payroll deduction between 1% and 15% of their compensation. The purchase price for shares of common stock purchased under the ESPP is 85% of the lesser of the fair market value of the Company’s common stock on (i) the first day of the applicable offering period or (ii) the last day of the purchase period in the applicable offering period. An employee’s participation automatically ends upon termination of employment for any reason.

The Company initially reserved 996,709 shares of common stock for sale under the ESPP. The aggregate number of shares reserved for sale under the ESPP increases automatically on each January 1, beginning January 1, 2019, by the number of shares equal to the least of (i) 1,000,000 shares of common stock, (ii) 1% of the aggregate number of shares of common stock outstanding on December 31st of the immediately preceding calendar year (rounded up to the nearest whole share), and (iii) an amount determined by the Board of Directors. No more than an aggregate of 10,102,525 shares of common stock may be issued over the ten-year term of the ESPP. As of December 31, 2021, 2,509,441 shares of common stock remain available for issuance under the ESPP.

As of December 31, 2021, there was approximately $0.4 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 August 1, 2021, and ended January 31, 2022.

For the years ended December 31, 2021, 2020, and 2019, the fair value of ESPP purchase rights was estimated using the Black-Scholes option-pricing model with the following assumptions:

		For the Year Ended December 31,
	2021		2020		2019
Fair market value of common stock	$152.35 to $166.84			$87.23 to $138.83		$40.60 to $82.12
Volatility	31% to 34%			32% to 49%		40%
Expected term	0.5 years			0.5 years		0.5 years
Expected dividend yield	n/a			n/a		n/a
Risk-free interest rate	0.06% to 0.08%			0.11% to 1.54%		2.04% to 2.59%