Stock-Based Compensation Stock-Based Compensation

6 Months Ended

Jun. 30, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation

Stock-Based Compensation

The Company allocates stock-based compensation expense related to equity plans for employees and non-employee directors among cost of sales, research and development and other engineering expense, selling expense, or general and administrative expense based on the job functions performed by the employees to whom the stock-based compensation is awarded. The Company recognized stock-based compensation expense related to its equity plans for employees of $2.5 million and $2.9 million for the three months ended June 30, 2019 and 2018, respectively, and $6.6 million and $6.0 million for the six months ended June 30, 2019 and 2018, respectively. Stock-based compensation cost capitalized in inventory was not material for all periods presented.

During the six months ended June 30, 2019, the Company granted 208,231 restricted stock units ("RSUs") to the Company's employees, including officers, and eight non-employee directors at an estimated weighted average fair value of $57.73 per share based on the closing price (adjusted for the present value of dividends) of the Company's common stock on the grant date. The RSUs granted to the Company's employees may be time-based, performance-based or time- and performance-based. Certain of the performance-based RSUs are granted to officers and key employees, where the number of performance-based awards to be issued is based on the achievement of certain Company performance criteria established in the RSU agreement over a cumulative three year period. These awards cliff vest after three years. In addition, these same officers and key employees also receive time-based RSUs, which vest pursuant to a three-year graded vesting schedule. Time- and performance based RSUs that are granted to the Company's employees excluding officers and certain key employees, vest ratably over the four year life of the award, and require the underlying shares of the Company's common stock to be subject to a performance-based adjustment during the first year of the award.

As of June 30, 2019, the Company's aggregate unamortized stock compensation expense was approximately $15.3 million, which is entirely attributable to unvested RSUs and is expected to be recognized in expense over a weighted-average period of 2.5 years.