SHARE-BASED COMPENSATION

3 Months Ended

Mar. 31, 2020

Share-based Payment Arrangement [Abstract]

As of March 31, 2020, approximately 4.5 million shares were reserved for future issuance under the Company's stock plans. A maximum of approximately 1.9 million of these shares can be awarded as restricted stock units (“RSUs”).

Stock Option Information

A summary of stock option activity under all stock plans for the three months ended March 31, 2020, is presented as follows (in millions, except per share amounts):


	Stock Options Outstanding
	Number Outstanding	Weighted Average Exercise Price Per Share
Balance at December 31, 2019	5.4	$	246.64

Options granted	0.2	$	539.01
Options exercised	(0.3)	$	170.76
Options forfeited/expired	(0.1)	$	458.77
Balance at March 31, 2020	5.2	$	262.89

As of March 31, 2020, options to purchase an aggregate of 4.1 million shares of common stock were exercisable at a weighted average price of $203.69 per share.

Restricted Stock Units Information

A summary of RSUs activity under all stock plans for the three months ended March 31, 2020, is presented as follows (in millions, except per share amounts):


	Shares	Weighted Average Grant Date Fair Value
Unvested balance at December 31, 2019	1.9	$	410.09
RSUs granted	0.6	$	535.79
RSUs vested	(0.6)	$	334.98
RSUs forfeited	—	$	441.33
Unvested balance at March 31, 2020	1.9	$	478.86

During the three months ended March 31, 2020, approximately 22,000 RSUs were forfeited.

Employee Stock Purchase Plan

Under the Employee Stock Purchase Plan (“ESPP”), employees purchased approximately 0.1 million shares for $36.6 million and approximately 0.1 million shares for $30.3 million during the three months ended March 31, 2020, and 2019, respectively.

Share-based Compensation Expense

The following table summarizes share-based compensation expense for the three months ended March 31, 2020, and 2019 (in millions):


	Three Months Ended March 31,
	2020		2019
Cost of sales – products	$	12.8	$	11.0
Cost of sales – services	5.5		4.5
Total cost of sales	18.3		15.5
Selling, general, and administrative	45.7		38.6
Research and development	27.2		22.8
Share-based compensation expense before income taxes	91.2		76.9
Income tax benefit	18.9		16.4
Share-based compensation expense after income taxes	$	72.3	$	60.5

The Black-Scholes option pricing model is used to estimate the fair value of stock options granted under the Company’s share-based compensation plans and rights to acquire stock granted under the ESPP. The weighted-average estimated fair values of stock options and the rights to acquire stock under the ESPP, as well as the weighted-average assumptions used in calculating the fair values of stock options and the rights to acquire stock under the ESPP that were granted during the three months ended March 31, 2020, and 2019, were as follows:


	Three Months Ended March 31,
	2020	2019
Stock Options
Risk-free interest rate	0.9%	2.5%
Expected term (in years)	4.3	4.3
Expected volatility	28%	31%
Fair value at grant date	$133.25	$157.64
ESPP
Risk-free interest rate	1.5%	2.5%
Expected term (in years)	1.1	1.2
Expected volatility	27%	31%
Fair value at grant date	$149.85	$154.20