Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2020

Weighted Average Assumptions of Fair Value Options Using Black-Scholes Option-Pricing Model

The fair value of each option grant is estimated on the date of grant using the Black-Scholes option-pricing model with the following weighted average assumptions for the years ended December 31, 2020, 2019 and 2018:


	December 31,
	2020	2019	2018
Dividend yield	—%	—%	—%
Risk-free interest rate	0.26% – 0.66%	1.66% – 3.05%	2.5% – 3.1%
Expected life (in years)	5.0	5.0	5.0
Historical volatility	46.50% – 53.91%	43.42% – 44%	43.5% – 48.4%

Schedule of Stock Option Activity

A summary of the Company’s stock option activity and weighted average exercise prices follows:


	Stock Option Activity			Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
	Options (in thousands)	Weighted Average Exercise Price








Balance outstanding at December 31, 2017	7,959	$	10.71
Granted	2,033	15.00
Exercised	(3,120)	10.70
Cancelled or expired	(606)	10.03
Balance outstanding at December 31, 2018	6,266	$	12.13	6.55	$	43,348
Options vested and expected to vest	5,550	$	11.89	6.28	$	39,521
Options exercisable at December 31, 2018	3,278	$	11.12	4.64	$	25,367

Balance outstanding at December 31, 2018	6,266	$	12.13
Granted	1,425	29.76
Exercised	(1,523)	11.12
Cancelled or expired	(369)	14.76
Balance outstanding at December 31, 2019	5,799	$	16.57	6.79	$	119,064
Options vested and expected to vest	5,096	$	15.29	6.49	$	110,934
Options exercisable at December 31, 2019	2,901	$	12.03	4.95	$	72,424

Balance outstanding at December 31, 2019	5,799	$	16.57
Granted	737	31.21
Exercised	(1,683)	12.69
Cancelled or expired	(521)	23.27
Balance outstanding at December 31, 2020	4,332	$	19.78	6.79	$	183,825
Options vested and expected to vest	1,470	$	23.88	8.19	$	56,382
Options exercisable at December 31, 2020	2,280	$	14.80	5.40	$	108,128

Schedule Restricted Stock Units Activity

Restricted Stock Unit Activity

A summary of the Company’s restricted stock units (“RSUs”) activity and weighted average exercise prices follows:


	Restricted Stock Unit Activity
	Number of Shares (in thousands)	Weighted Average Grant Date Fair Value (Per Share)		Aggregate Fair Value (in thousands)






Balance outstanding at December 31, 2017	873	$	8.29	$	10,053
Awarded	2,568	17.02		—
Released	(361)	9.49		—
Forfeited	(390)	9.49		—
Non-vested and outstanding at December 31, 2018	2,690	$	15.81	$	50,756

Balance outstanding at December 31, 2018	2,690	$	15.81	$	50,756
Awarded	1,979	30.99		—
Released	(1,197)	14.24		—
Forfeited	(423)	20.28		—
Non-vested and outstanding at December 31, 2019	3,049	$	24.73	$	112,848


Balance outstanding at December 31, 2019	3,049	$	24.73	$	112,848
Awarded	2,530	26.51		—
Released	(1,906)	23.40		—
Forfeited	(723)	25.19		—
Non-vested and outstanding at December 31, 2020	2,950	$	27.00	$	183,781
Expected to vest	1,939	$	26.17	$	120,674