The following table summarizes stock-based compensation expense by financial statement line item in the Company’s consolidated statements of operations for the three and nine months ended December 31, 2018 and 2017:

	For the Three Months Ended December 31,				For the Nine Months Ended December 31,
		2018		2017		2018		2017
	(in $000's)
Cost of product revenue	$	773	$	489	$	2,122	$	1,221
Research and development		2,638		1,673		7,222		4,217
Selling, general and administrative		12,167		9,070		34,413		23,732
	$	15,578	$	11,232	$	43,757	$	29,170

The following table summarizes the stock option activity for the nine months ended December 31, 2018:

						Weighted
				Weighted		Average		Aggregate
				Average		Remaining		Intrinsic
	Options			Exercise		Contractual		Value
	(in thousands)			Price		Term (years)		(in thousands)
Outstanding at beginning of period		1,282		$	46.18		5.31
Granted		78			381.62
Exercised		(435	)		25.36
Cancelled and expired		(19	)		145.89
Outstanding at end of period		906		$	83.90		5.74	$	222,852
Exercisable at end of period		668		$	39.86		4.78	$	190,646
Options vested and expected to vest at end of period		896		$	83.73		5.72	$	220,460

Stock options generally vest and become exercisable annually over three years. The remaining unrecognized stock-based compensation expense for unvested stock option awards at December 31, 2018 was approximately $14.0 million and the estimated weighted-average period over which this cost will be recognized is 2.1 years.

The aggregate intrinsic value of options exercised was $158.6 million for the nine months ended December 31, 2018. The total cash received as a result of employee stock options exercised during the nine months ended December 31, 2018 was approximately $11.0 million.

The Company estimates the fair value of stock options granted at the grant date using the Black-Scholes option valuation model. The weighted average grant-date fair values and weighted average assumptions used in the calculation of fair value of stock options granted during the three and nine months ended December 31, 2018 and 2017 was as follows:

	For the Three Months Ended December 31,						For the Nine Months Ended December 31,
		2018			2017			2018			2017
Weighted average grant-date fair value	$	117.50		$	70.14		$	142.08		$	51.20

Valuation assumptions:
Risk-free interest rate		2.81	%		2.13	%		2.93	%		1.86	%
Expected option life (years)		4.04			4.08			4.04			4.07
Expected volatility		45.2	%		42.2	%		42.8	%		43.6	%

The following table summarizes activity of restricted stock units for the nine months ended December 31, 2018:

	Number of Shares			Weighted Average Grant Date Fair Value
	(in thousands)			(per share)
Restricted stock units at beginning of period		880		$	109.01
Granted		193		$	379.30
Vested		(418	)	$	103.66
Forfeited		(25	)	$	187.05
Restricted stock units at end of period		630		$	192.16

Restricted stock units generally vest annually over three years. The remaining unrecognized compensation expense for outstanding restricted stock units, including performance and market-based awards, as of December 31, 2018 was $60.9 million and the estimated weighted-average period over which this cost will be recognized is 2.1 years.

The weighted average grant-date fair value for restricted stock units granted during the nine months ended December 31, 2018 was $379.30. The total fair value of restricted stock units vested during the nine months ended December 31, 2018 was $168.1 million.

In May 2018, performance-based awards of restricted stock units for the potential issuance of approximately 114,000 shares of common stock were issued to certain executive officers and employees, which vest upon achievement of prescribed service milestones by the award recipients and performance milestones by the Company. As of December 31, 2018, the Company is recognizing compensation expense based on the probable outcome related to the prescribed performance targets on the outstanding awards.