STOCK-BASED EMPLOYEE COMPENSATION

Stock-based compensation expense included in personnel expenses in the Unaudited Consolidated Statements of Operations was $74 million and $71 million for the three months ended March 31, 2019 and 2018, respectively.

Stock-based compensation expense is recognized in the consolidated financial statements based upon fair value. Fair value is recognized as an expense on a straight-line basis over the employee's requisite service period and forfeitures are accounted for when they occur. The fair value on the grant date of performance share units and restricted stock units is determined based on the number of units granted and the quoted price of the Company's common stock. Stock-based compensation expense related to performance share units reflects the estimated probable outcome at the end of the performance period. The fair value of employee stock options assumed in acquisitions was determined using the Black-Scholes model and the market value of the Company's common stock at the respective acquisition dates.

Restricted Stock Units and Performance Share Units

The following table summarizes the activity of restricted stock units and performance share units ("share-based awards") during the three months ended March 31, 2019:



Share-Based Awards	Shares		Weighted-average Grant Date Fair Value
Unvested at December 31, 2018	511,562			$	1,713
Granted	192,775			$	1,714
Vested	(172,432	)		$	1,443
Performance shares adjustment	127			$	1,580
Forfeited/Canceled	(8,093	)		$	1,823
Unvested at March 31, 2019	523,939			$	1,802

At March 31, 2019, there was $663 million of total future compensation cost related to unvested share-based awards to be recognized over a weighted-average period of 2.3 years.

During the three months ended March 31, 2019, the Company made broad-based grants of 131,683 restricted stock units that generally vest over a three-year period, subject to certain exceptions for terminations other than for "cause," for "good reason" or on account of death or disability. These share-based awards had a total grant date fair value of $226 million based on a weighted-average grant-date fair value per share of $1,714.

Performance share units are payable in shares of the Company's common stock upon vesting. Subject to certain exceptions for terminations other than for "cause," for "good reason" or on account of death or disability, recipients of these performance share units generally must continue their service through the requisite service period in order to receive any shares. Stock-based compensation related to performance share units reflects the estimated probable outcome at the end of the performance period.

2019 Performance Share Units

During the three months ended March 31, 2019, the Company granted 61,092 performance share units to executives and certain other employees. The performance share units had a total grant-date fair value of $105 million based on a weighted-average grant-date fair value per share of $1,714. The actual number of shares to be issued on the vesting date will be determined upon completion of the performance period which generally ends December 31, 2021, assuming there is no accelerated vesting for, among other things, a termination of employment under certain circumstances. At March 31, 2019, the estimated number of probable shares to be issued is a total of 61,092 shares, net of performance share units that were forfeited or vested since the grant date, including 47,674 shares that are not subject to the achievement of minimum performance thresholds. If the maximum performance thresholds are met at the end of the performance period, a maximum number of 122,184 total shares could be issued.

2018 Performance Share Units

During the year ended December 31, 2018, the Company granted 49,721 performance share units with a grant-date fair value of $101 million, based on a weighted-average grant-date fair value per share of $2,034. The actual number of shares to be issued will be determined upon completion of the performance period which generally ends December 31, 2020, assuming there is no accelerated vesting for, among other things, a termination of employment under certain circumstances.

At March 31, 2019, there were 44,900 unvested 2018 performance share units outstanding, net of performance share units that were forfeited or vested since the grant date. At March 31, 2019, the number of shares estimated to be issued pursuant to these performance share units at the end of the performance period is a total of 86,157 shares, including 33,590 shares that are not subject to the achievement of minimum performance thresholds. If the maximum performance thresholds are met at the end of the performance period, a maximum of 89,800 shares could be issued pursuant to these performance share units.

2017 Performance Share Units

During the year ended December 31, 2017, the Company granted 73,893 performance share units with a grant-date fair value of $128 million, based on a weighted-average grant date fair value per share of $1,735. The actual number of shares to be issued will be determined upon completion of the performance period which generally ends December 31, 2019, assuming there is no accelerated vesting for, among other things, a termination of employment under certain circumstances.

At March 31, 2019, there were 56,506 unvested 2017 performance share units outstanding, net of performance share units that were forfeited or vested since the grant date. At March 31, 2019, the number of shares estimated to be issued pursuant to these performance share units at the end of the performance period is a total of 88,098 shares, including 45,843 shares that are not subject to the achievement of minimum performance thresholds. If the maximum thresholds are met at the end of the performance period, a maximum of 113,012 shares could be issued pursuant to these performance share units.

Stock Options

All outstanding employee stock options were assumed in acquisitions. The following table summarizes the activity for stock options during the three months ended March 31, 2019:



Employee Stock Options	Number of Shares		Weighted-average Exercise Price			Aggregate Intrinsic Value (in millions)			Weighted-average Remaining Contractual Term (in years)
Balance, December 31, 2018	27,263			$	387		$	36	2.8
Exercised	(1,164	)		$	317
Balance, March 31, 2019	26,099			$	404		$	35	2.5
Vested and exercisable at March 31, 2019	26,099			$	404		$	35	2.5

The aggregate intrinsic value of employee stock options exercised during the three months ended March 31, 2019 and 2018 was $2 million and $4 million, respectively. During the three months ended March 31, 2019 and 2018, stock options vested for 73 and 83 shares, respectively.