The Company has granted non-qualified and incentive stock options, restricted stock units and restricted stock awards under its incentive plans. The Company recognizes compensation expense based on estimated grant date fair values for all stock-based awards issued to employees and directors, including stock options, restricted stock awards and restricted stock units.

The total stock-based compensation expense related to all stock-based awards was $21.0 million and $20.2 million during the three months ended March 31, 2021 and 2020, respectively. As of March 31, 2021, $227.9 million of total unrecognized stock-based compensation expense is expected to be recognized over a weighted-average period of 2.9 years.

Excess tax deficiencies reflect the total realized value of the Company’s tax deductions from individual stock option exercise transactions and the vesting of restricted stock units deficient of the deferred tax assets that were previously recorded. During the three months ended March 31,2020, the Company recognized excess tax deficiencies from stock-based compensation of $2.5 million within income tax benefit in the condensed consolidated statements of operations and within cash flows from operating activities in the condensed consolidated statements of cash flows. Excess tax benefits (deficiencies) had no impact on income tax benefit during the three months ended March 31, 2021.

The Company capitalized stock-based compensation expense as website and software development costs of $4.6 million and $4.8 million during the three months ended March 31, 2021 and 2020, respectively.

The Company granted 16,841 and 224,509 stock options under the 2015 Plan during the three months ended March 31, 2021 and 2020, respectively. The fair value of each stock option award was estimated based on the assumptions below as of the grant date using the Black-Scholes-Merton option pricing model. Expected volatility is based on the historical and implied volatilities of the Company’s own common stock. The Company uses historical data to estimate option exercises and employee terminations within the valuation model. Separate groups of employees that have similar historical exercise behavior are considered separately for valuation purposes. The expected term calculation for option awards considers a combination of the Company’s historical and estimated future exercise behavior. The risk-free rate for the period within the contractual life of the option is based on the U.S. Treasury yield curve in effect at the time of grant.

The assumptions used to determine the fair value of the stock options granted during the three months ended March 31, 2021 and 2020 were as follows:

	Three Months Ended March 31,
	2021			2020
Weighted-average fair value options granted	$	31.02		$	19.67
Average risk-free interest rate		0.33	%		1.44	%
Expected stock price volatility		55.9	%		48.3	%
Dividend yield	None			None
Expected stock option life (years)		4.00			4.00

Stock option awards as of December 31, 2020 and March 31, 2021, and changes during the three months ended March 31, 2021, were as follows:

	Options			Weighted-Average Exercise Price		Aggregate Intrinsic Value (thousands)		Weighted-Average Exercise Term (years)
Outstanding at December 31, 2020		2,511,570		$	41.58	$	88,030		5.80
Granted		16,841			72.54
Forfeited		(2,433	)		26.34
Exercised		(34,204	)		32.24
Outstanding at March 31, 2021		2,491,774			41.93		59,508		5.60
Vested and expected to vest at March 31, 2021		2,452,818			41.60		59,267		5.55
Exercisable at March 31, 2021		2,065,269		$	36.98	$	57,112		5.07

The aggregate intrinsic value in the table above represents the total pre-tax intrinsic value (the difference between the fair value of the common stock and the exercise price, multiplied by the number of in-the-money options) that would have been received by the option holders had all option holders exercised their in-the-money options on each date. This amount will change in future periods based on the fair value of the Company’s stock and the number of options outstanding. The aggregate intrinsic value of awards exercised was $1.4 million and $2.0 million during the three months ended March 31, 2021 and 2020, respectively.

The Company recorded compensation expense for stock options of $2.1 million and $3.2 million for the three months ended March 31, 2021 and 2020, respectively. As of March 31, 2021, total unrecognized compensation cost, adjusted for estimated forfeitures, related to non-vested stock options was $11.0 million and is expected to be recognized over a weighted-average period of 1.8 years.

Non-vested restricted stock units as of December 31, 2020 and March 31, 2021, and changes during the three months ended March 31, 2021 were as follows:

	Restricted Stock Units
	Shares			Weighted-Average Grant Date Fair Value
Outstanding at December 31, 2020		3,088,736		$	61.47
Granted		1,456,877			72.56
Forfeited		(156,491	)		63.93
Vested		(347,836	)		61.71
Outstanding at March 31, 2021		4,041,286		$	65.35

Compensation expense related to restricted stock units was $18.9 million and $17.0 million during the three months ended March 31, 2021 and 2020, respectively. The aggregate fair value as of the vest date of restricted stock units that vested during the three months ended March 31, 2021 and 2020 was $25.6 million and $22.5 million, respectively. As of March 31, 2021, $216.9 million of total unrecognized compensation cost, adjusted for estimated forfeitures, related to 3,580,526 non-vested restricted stock units expected to vest with weighted-average grant date fair values of $65.44, is expected to be recognized over a weighted-average period of 2.9 years. The fair value of these awards was determined based on the Company’s stock price at the grant date and assumes no expected dividend payments through the vesting period.