STOCK-BASED EMPLOYEE COMPENSATION

Stock-based compensation expense included in personnel expenses in the Unaudited Consolidated Statements of Operations was approximately $74.5 million and $66.9 million for the three months ended June 30, 2018 and 2017, respectively, and $145.9 million and $125.8 million for the six months ended June 30, 2018 and 2017, respectively.

Stock-based compensation expense is recognized in the 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 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 as of the grant date. 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 six months ended June 30, 2018:



Share-Based Awards	Shares		Weighted-Average Grant Date Fair Value
Unvested at December 31, 2017	524,696			$	1,431.88
Granted	153,637			$	2,035.68
Vested	(184,648	)		$	1,277.74
Performance Shares Adjustment	12,676			$	1,587.94
Forfeited/Canceled	(21,685	)		$	1,649.68
Unvested at June 30, 2018	484,676			$	1,676.34

At June 30, 2018, there was $496.1 million of total future compensation cost related to unvested share-based awards to be recognized over a weighted-average period of 2.1 years.

During the six months ended June 30, 2018, the Company made broad-based grants of 103,916 restricted stock units that generally have a three-year vesting 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 $211.6 million based on a weighted-average grant date fair value per share of $2,036.58.

In addition, during the six months ended June 30, 2018, the Company granted 49,721 performance share units to executives and certain other employees. The performance share units had a total grant date fair value of $101.1 million based on a weighted-average grant date fair value per share of $2,033.79. The 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. 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, 2020, assuming there is no accelerated vesting for, among other things, a termination of employment under certain circumstances. At June 30, 2018, the estimated number of probable shares to be issued is a total of 49,475 shares, net of performance share units that were forfeited or vested since the grant date. If the maximum performance thresholds are met at the end of the performance period, a maximum number of 98,950 total shares could be issued. If the minimum performance thresholds are not met, 38,165 shares would be issued at the end of the performance period.

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 $128.2 million, based on a grant date fair value per share of $1,735.10. 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 June 30, 2018, there were 64,769 unvested 2017 performance share units outstanding, net of performance share units that were forfeited or vested since the grant date. At June 30, 2018, 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 84,507 shares. If the maximum performance thresholds are met at the end of the performance period, a maximum of 129,538 total shares could be issued pursuant to these performance share units. If the minimum performance thresholds are not met, 51,512 shares would be issued at the end of the performance period.

2016 Performance Share Units

During the year ended December 31, 2016, the Company granted 85,735 performance share units with a grant date fair value of $111.7 million, based on a weighted-average grant date fair value per share of $1,302.25. The actual number of shares to be issued will be determined upon completion of the performance period which generally ends December 31, 2018.

At June 30, 2018, there were 66,270 unvested 2016 performance share units outstanding, net of performance share units that were forfeited or vested since the grant date. At June 30, 2018, 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 111,196 shares. If the maximum thresholds are met at the end of the performance period, a maximum of 148,953 total shares could be issued pursuant to these performance share units. If the minimum performance thresholds are not met, 39,498 shares would be issued at the end of the performance period.

Stock Options

All outstanding employee stock options were assumed in acquisitions. The following table summarizes the activity for stock options during the six months ended June 30, 2018:



Employee Stock Options	Number of Shares		Weighted-Average Exercise Price			Aggregate Intrinsic Value (in thousands)			Weighted-Average Remaining Contractual Term (in years)
Balance, December 31, 2017	30,675			$	401.61		$	40,986	3.9
Exercised	(2,770	)		$	480.63
Forfeited	(13	)		$	241.83
Balance, June 30, 2018	27,892			$	393.92		$	45,552	3.3
Vested and exercisable at June 30, 2018	27,819			$	402.22		$	45,492	3.3
Vested and exercisable at June 30, 2018 and expected to vest thereafter	27,892			$	393.92		$	45,552	3.3

The aggregate intrinsic value of employee stock options exercised during the six months ended June 30, 2018 and 2017 was $4.4 million and $16.8 million, respectively. During the six months ended June 30, 2018 and 2017, stock options vested for 98 and 954 shares, respectively.