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Stock-based Compensation
 3 Months Ended
Mar. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-based Compensation 7.
STOCK-BASED COMPENSATION
Stock Unit Awards
Along with cash-based salaries and performance-based annual cash incentive awards, stock unit awards represent an important element of compensation to our employees. Restricted stock unit ("RSU") and performance stock unit ("PSU") awards activity is presented in the following tables.
 
 
Shares (thousands)
 
Weighted Average
Grant Date
Fair Value
Weighted Average
Remaining
Contractual Life
(in years)
Unvested as of December 31, 2018
652.7

 
$
131.32

2.02
Granted
74.7

 
152.84

 
Vested
(195.5
)
 
114.79

 
Forfeited
(4.9
)
 
126.70

 
Unvested as of March 31, 2019
527.0

 
$
140.55

2.15
 
 
 
 
 
Unvested as of December 31, 2017
727.7

 
$
118.96

1.24
Granted
119.3

 
160.86

 
Vested
(173.8
)
 
121.15

 
Forfeited
(15.2
)
 
124.62

 
Unvested as of March 31, 2018
658.0

 
$
126.31

2.20

We determine the fair value of RSUs, subject only to service requirements, based on the closing market price of our common stock on the grant date. PSUs are subject to service requirements and one or more performance measures, including (i) performance conditions (e.g. achievement against earnings per share targets) and (ii) for certain awards, a market condition (e.g. total shareholder return performance against a peer group). We determine the fair value of PSUs based on the closing market price of our common stock on the grant date taking into consideration the likelihood of achieving each performance condition and the market condition valuation, as applicable, based on the output of a Monte Carlo simulation.
As of March 31, 2019, we had $39.1 million of unamortized deferred compensation related to unvested restricted stock units, which we anticipate recognizing over varying periods into 2023.