(4) STOCK-BASED COMPENSATION

Stock-Based Compensation Plans

The Company maintains the 2010 Omnibus Long-Term Incentive Plan (As Amended and Restated Effective July 27, 2017), the 2010 Employee Stock Purchase Plan, the 2015 Inducement Award Plan, the 2016 Inducement Award Plan and the 2000 Stock Option and Incentive Plan (collectively, the “Stock Plans”).

Stock-Based Compensation Expense

The Company records stock-based compensation expense in connection with the amortization of restricted stock and restricted stock unit awards, stock purchase rights granted under the Company’s employee stock purchase plan and stock options granted to employees, non-employee consultants and non-employee directors. The Company recorded $12.5 million in stock-based compensation expense during the three months ended March 31, 2018. The Company recorded $6.1 million in stock-based compensation expense during the three months ended March 31, 2017.

Determining Fair Value

Valuation and Recognition – The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model. The fair value of each market measure-based award is estimated on the date of grant using a Monte Carlo simulation pricing model. The fair value of service-based awards for each restricted stock unit award is determined on the date of grant using the closing stock price on that day. The estimated fair value of these awards is recognized to expense using the straight-line method over the vesting period. The Black-Scholes and Monte Carlo pricing models utilize the following assumptions:

Expected Term – Expected life of an option award is the average length of time over which the Company expects employees will exercise their options, which is based on historical experience with similar grants. Expected life of a market measure-based award is based on the applicable performance period.

Expected Volatility - Expected volatility is based on the Company’s historical stock volatility data over the expected term of the awards.

Risk-Free Interest Rate - The Company bases the risk-free interest rate used in the Black-Scholes and Monte Carlo valuation models on the implied yield currently available on U.S. Treasury zero-coupon issues with an equivalent expected term.

Forfeitures – Beginning in 2017, the Company adopted Accounting Standards Update (“ASU”) No. 2016-09, Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting (“Update 2016-09”). With the adoption of Update 2016-09, forfeiture estimates are no longer required, and the effects of actual forfeitures are recorded at the time they occur. The impact on the condensed consolidated balance sheet as of March 31, 2017 was a cumulative-effect adjustment of $0.4 million, increasing opening accumulated deficit and additional paid-in capital.

The fair value of each option and market measure-based award is based on the assumptions in the following table:


	Three Months Ended
	March 31,
	2018	2017
Option Plan Shares
Risk-free interest rates	2.79%	2.13%
Expected term (in years)	6.44	6.59
Expected volatility	61.8%	62.9%
Dividend yield	0%	0%
Weighted average fair value per share of options granted during the period	$26.84	$13.20

Stock Option and Restricted Stock Activity

A summary of stock option activity under the Stock Plans during the three months ended March 31, 2018 is as follows:


				Weighted
		Weighted		Average
		Average		Remaining	Aggregate
		Exercise		Contractual	Intrinsic
Options	Shares	Price		Term (Years)	Value(1)
(Aggregate intrinsic value in thousands)
Outstanding, January 1, 2018	3,360,461	$	11.89	6.4
Granted	343,566		44.37
Exercised	(420,129)		3.31
Forfeited	—		—
Outstanding, March 31, 2018	3,283,898	$	16.39	7.0	$	80,098
Exercisable, March 31, 2018	1,757,384	$	10.80	5.4	$	51,901

(1)

The aggregate intrinsic value of options outstanding, exercisable and vested and expected to vest is calculated as the difference between the exercise price of the underlying options and the market price of the Company’s common stock for options that had exercise prices that were lower than the $40.33 market price of the Company’s common stock at March 31, 2018. The total intrinsic value of options exercised during the three months ended March 31, 2018 and 2017 was $19.1 million and $0.2 million, respectively.

As of March 31, 2018, there was $137.6 million of total unrecognized compensation cost related to non-vested share-based compensation arrangements granted under all Stock Plans. Total unrecognized compensation cost will be adjusted for future forfeitures. The Company expects to recognize that cost over a weighted average period of 3.2 years.

A summary of restricted stock and restricted stock unit activity under the Stock Plans during the three months ended March 31, 2018 is as follows:


		Weighted
	Restricted	Average Grant
	Shares	Date Fair Value
Outstanding, January 1, 2018	6,148,778	$	15.76
Granted	1,079,223		45.16
Released	(880,022)		17.67
Forfeited	(33,307)		26.37
Outstanding, March 31, 2018	6,314,672	$	20.36