8. SHARE-BASED COMPENSATION

The compensation expense related to the Company’s share-based compensation arrangements has been included in the condensed consolidated statements of comprehensive loss as follows (in millions):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Sales, general and administrative	$	6.5	$	3.5	$	18.2	$	11.8
Research and development		3.7		2.8	$	10.9	$	8.7

Total share-based compensation expense	$	10.2	$	6.3	$	29.1	$	20.5

The fair value of equity instruments that vest based on continued employee service, net of estimated forfeitures, is recognized and amortized on a straight-line basis over the requisite service period. For restricted stock units (RSUs) with performance-based vesting requirements (PRSUs), no expense is recorded until the performance condition is probable of being achieved. The Company estimates forfeiture rates for equity awards based on past behavior for similar equity awards with further consideration given to the class of employees to whom the equity awards were granted.

As of September 30, 2017, total unrecognized estimated compensation cost related to non-vested stock options and non-vested RSUs, that vest over a given service period, granted prior to that date was $50.9 million and $32.7 million, respectively, which is expected to be recognized over a weighted average period of approximately 2.8 years and 2.9 years, respectively. Additionally, the Company has approximately 0.2 million PRSUs outstanding. The total unrecognized estimated compensation cost related to these PRSUs is $7.5 million and will be recognized over the expected performance period once the achievement of performance conditions becomes probable.

During the nine months ended September 30, 2017 and 2016, stock options to purchase approximately 1.0 million and 0.3 million shares of the Company’s common stock were exercised, respectively. The cash received by the Company from stock option exercises during the nine months ended September 30, 2017 and 2016 was approximately $8.3 million and $2.3 million, respectively. The Company also issued approximately 0.3 million shares of common stock pursuant to the vesting of RSUs during each of the nine months ended September 30, 2017 and 2016, and 0.2 million shares of common stock pursuant to the vesting of PRSUs during the second quarter of 2017.

Stock Option Assumptions

The Company granted stock options to purchase approximately 1.6 million and 1.0 million shares of the Company’s common stock during the nine months ended September 30, 2017 and 2016, respectively. These stock options generally vest monthly over a four-year period. The exercise price of all stock options granted during the nine months ended September 30, 2017 and 2016 was equal to the closing price of the Company’s common stock on the date of grant. The estimated fair value of each stock option granted was determined on the date of grant using the Black-Scholes option-pricing model with the following weighted-average assumptions for the stock option grants:


	Three Months Ended September 30,						Nine Months Ended September 30,
	2017			2016			2017			2016
Risk-free interest rate		1.8	%		1.1	%		2.0	%		1.4	%
Expected volatility of common stock		59.6	%		59.3	%		58.2	%		60.0	%
Dividend yield		0.0	%		0.0	%		0.0	%		0.0	%
Expected option term		5 years			5 years			5.7 years			5.6 years

The Black-Scholes option-pricing model incorporates various and highly sensitive assumptions including expected volatility, expected term and interest rates. The expected volatility is based on the historical volatility of the Company’s common stock over the most recent period commensurate with the estimated expected term of the Company’s stock options. The expected option term is estimated based on historical experience as well as the status of the employee. For example, directors and officers have a longer expected option term than all other employees. The risk-free rate for periods within the contractual life of the option is based upon observed interest rates appropriate for the expected term of the Company’s employee stock options. The Company has never declared or paid dividends and has no plans to do so in the foreseeable future. For the nine months ended September 30, 2017 and 2016, share-based compensation expense related to stock options was $18.7 million and $13.1 million, respectively.

Restricted Stock Units

During the nine months ended September 30, 2017 and 2016, the Company granted approximately 0.6 million and 0.3 million RSUs, respectively, that generally vest annually over a four-year period. Additionally, during the nine months ended September 30, 2016, the Company granted approximately 0.2 million PRSUs. These PRSUs vest based on the achievement of pre-defined Company-specific performance criteria and expire four years from the grant date. Expense recognition for PRSUs commences when attainment of the performance based criteria is determined to be probable. The fair value of RSUs and PRSUs is estimated based on the closing sale price of the Company’s common stock on the date of grant. For the nine months ended September 30, 2017 and 2016, share-based compensation expense related to RSUs and PRSUs was $10.4 million and $7.4 million, respectively.