STOCK-BASED EMPLOYEE COMPENSATION (Tables)

3 Months Ended

Mar. 31, 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 three months ended March 31, 2021: 

Restricted Stock Units	Shares	Weighted-average Grant-date Fair Value
Unvested at December 31, 2020	305,959		$	1,697
Granted	119,284		$	2,275
Vested	(103,989)		$	1,811
Forfeited	(12,143)		$	1,761
Unvested at March 31, 2021	309,111		$	1,880

Activity of Performance Share Units

The following table summarizes the activity of performance share units for employees during the three months ended March 31, 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	(51,354)		$	2,022
Performance shares adjustment (2)	42,298		$	2,188
Forfeited	(4,651)		$	1,756
Unvested at March 31, 2021	112,944		$	2,107

(1) Excludes 12,251 performance share units awarded during the three months ended March 31, 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 March 31, 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	64,395	9,492	39,057
Shares not subject to the achievement of minimum performance thresholds	28,858	—	39,057
Shares that could be issued if maximum performance thresholds are met	64,395	18,080	73,687

(1) Excludes performance share units awarded during the three months ended March 31, 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 three months ended March 31, 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	(655)		$	1,094
Forfeited	(7,275)		$	1,411
Balance, March 31, 2021	144,816		$	1,402		$	134	9.0
Exercisable at March 31, 2021	4,230		$	1,217		$	5	5.6