STOCK-BASED EMPLOYEE COMPENSATION (Tables)

6 Months Ended

Jun. 30, 2021

Share-based Payment Arrangement [Abstract]

Activity of Restricted Stock Units

The following table summarizes the activity of restricted stock units for employees and non-employee directors during the six months ended June 30, 2021: 

Restricted Stock Units	Shares	Weighted-average Grant-date Fair Value
Unvested at December 31, 2020	305,959		$	1,697
Granted	123,939		$	2,275
Vested	(110,550)		$	1,789
Forfeited	(25,308)		$	1,836
Unvested at June 30, 2021	294,040		$	1,895

Activity of Performance Share Units

The following table summarizes the activity of performance share units for employees during the six months ended June 30, 2021:

Performance Share Units	Shares	Weighted-average Grant-date Fair Value
Unvested at December 31, 2020	84,478		$	1,930
Granted (1)	42,173		$	2,287
Vested	(55,426)		$	1,999
Performance shares adjustment (2)	44,449		$	2,125
Forfeited	(6,649)		$	1,773
Unvested at June 30, 2021	109,025		$	2,123

(1) Excludes 12,251 performance share units awarded during the six months ended June 30, 2021 for which the grant date under Accounting Standards Codification ("ASC") 718, Compensation - Stock Compensation, has not yet been established. Amongst other conditions, for the grant date to be established, a mutual understanding is required to be reached between the Company and the employee of the key terms and conditions of the award, including the performance targets. The performance targets for each of the annual performance periods under the award are set at the beginning of the respective year.

(2) Probable outcome for performance-based awards is updated based upon changes in actual and forecasted operating results or expected achievement of performance goals, as applicable, and the impact of modifications.

Estimated Vesting of Performance Share Units Granted

The following table summarizes the estimated vesting, as of June 30, 2021, of performance share units granted in 2021, 2020 and 2019, net of forfeiture and vesting since the respective grant dates:

Performance Share Units, by grant year	2021(1)	2020	2019
Shares probable to be issued	63,934	11,752	33,339
Shares not subject to the achievement of minimum performance thresholds	28,506	—	33,339
Shares that could be issued if maximum performance thresholds are met	63,934	18,080	62,251

(1) Excludes performance share units awarded during the six months ended June 30, 2021 for which the grant date under ASC 718 has not yet been established as disclosed above.

Activity for Stock Options

The following table summarizes the activity for stock options during the six months ended June 30, 2021: 

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, 2020	152,746		$	1,401		$	126	9.3
Exercised	(3,491)		$	1,248
Forfeited	(10,677)		$	1,411
Balance, June 30, 2021	138,578		$	1,405		$	109	8.7
Exercisable at June 30, 2021	1,394		$	767		$	2	2.1