Common Stock and Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2020

Common Stock and Stockholders'Equity

Summary of stock option activity under the Plan

Activity under the Plans is as follows (in thousands, except share and per share amounts and years):

				Weighted-
		Weighted-		Average
	Number of	Average		Remaining	Aggregate
	Shares	Exercise		Contractual	Intrinsic
	Outstanding	Price		Life in Years	Value
Balance at December 31, 2019	5,206,981	$	24.47	7.03	$	308,538
Stock option grants	10,832	$	125.65	0	$	0
Stock options exercised	(1,598,963)	$	21.00	0	$	0
Stock options forfeited	(148,655)	$	32.82	0	$	0
Balance at June 30, 2020	3,470,195	$	25.97	6.62	$	575,590
Vested or expected to vest at June 30, 2020	3,470,194	$	25.97	6.62	$	575,590
Exercisable at June 30, 2020	2,439,319	$	20.94	6.28	$	414,445

Assumptions used for estimate of fair value of options

	Six Months Ended June 30,
	2020		2019
Volatility		46.1% – 51.0%		46.9% – 47.6%
Expected life (in years)		4.3		5.2
Risk-free interest rate		0.27% - 1.64%		1.75% - 2.55%
Dividend yield		0		0
Weighted-average fair value of underlying stock options	$	49.44	$	27.34

Schedule of activity under the RSUs

Activity under the RSUs is as follows:

		Weighted-Average
		Grant Date
	RSUs	Fair Value Per RSU
Balance at December 31, 2019	1,483,558	$	54.13
Granted	342,383	$	125.69
Vested and issued	(565,724)	$	52.11
Forfeited	(73,072)	$	52.17
Balance at June 30, 2020	1,187,145	$	58.82
Vested and unissued at June 30, 2020	13,755	$	50.90
Non-vested at June 30, 2020	1,173,390	$	58.91

Schedule of activity under the PSUs

Activity under the PSU is as follows:

		Weighted-Average
		Grant Date
	Shares	Fair Value Per Share
Balance at December 31, 2019	512,482	$	62.51
Granted	111,199	$	117.81
Vested and issued	(186,455)	$	62.02
Balance at June 30, 2020	437,226	$	76.78
Vested and unissued at June 30, 2020	0	$	0
Non-vested at June 30, 2020	437,226	$	76.78

Components of operating expense charged for compensation cost expense

Total compensation costs charged as an expense for stock-based awards, including stock options, RSUs, PSUs and ESPP, recognized in the components of operating expenses are as follows (in thousands):

	Quarter Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
Administrative and marketing	$	1,544	$	1,317	$	2,803	$	2,138
Sales		3,271		2,528		6,190		4,658
Technology and development		2,559		2,141		4,663		4,039
General and administrative		14,554		11,382		26,587		20,056
Total stock-based compensation expense	$	21,928	$	17,368	$	40,243	$	30,891