The Company recorded stock-based compensation expense as follows in the unaudited condensed consolidated statements of income and comprehensive income (dollars in millions):

In addition to the awards above, the Company recorded stock-based compensation expense within other charges, net of $0.3 million and $0.4 million in the three months ended March 31, 2020 and 2019, respectively, related to the 2018 acquisition of Mestrelab Research, S.L. (Mestrelab).

Stock-based compensation expense is recognized on a straight-line basis over the underlying requisite service period of the stock-based award.

Stock options to purchase the Company's common stock are periodically awarded to executive officers and other employees of the Company subject to a vesting period of three to four years. The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model. There were no stock options awarded during the three months ended March 31, 2020 and 2019.

(a)	In addition to the options that are vested at March 31, 2020, the Company expects a portion of the unvested options to vest in the future. Options expected to vest in the future are determined by applying an estimated forfeiture rate to the options that are unvested as of March 31, 2020.

(b)	The aggregate intrinsic value is based on the positive difference between the fair value of the Company’s common stock price of $35.86 on March 31, 2020 and the exercise price of the underlying stock options.

The total intrinsic value of options exercised was $0.3 million and $3.1 million for the three months ended March 31, 2020 and 2019, respectively.

Restricted stock unit activity for the three months ended March 31, 2020 was as follows:


		Weighted
		Average Grant
	Shares Subject	Date Fair
	to Restriction	Value
Outstanding at December 31, 2019	865,101	$	34.73
Granted	23,859		50.70
Vested	(41,738)		30.40
Forfeited	(21,845)		32.65
Outstanding at March 31, 2020	825,377	$	35.46

The total fair value of restricted stock units vested was $1.3 million and $1.1 million for the three months ended March 31, 2020 and 2019, respectively.

At March 31, 2020, the Company expects to recognize pre-tax stock-based compensation expense of $2.4 million associated with outstanding stock option awards granted under the Company's stock plans over the weighted average remaining service period of 1.3 years. The Company also expects to recognize additional pre-tax stock-based compensation expense of $21.1 million associated with outstanding restricted stock units granted under the 2016 Plan over the weighted average remaining service period of 2.4 years.