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Stock-based Compensation
 3 Months Ended
Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Stock-based Compensation
7.
STOCK-BASED COMPENSATION
Restricted Stock Unit Awards
Along with cash-based salaries and performance-based annual cash incentive awards, restricted stock unit awards represent an important element of compensation to our employees. Restricted stock unit 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, 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
 
 
 
 
 
Unvested as of December 31, 2016
750.9

 
$
113.97

1.71
Granted
140.9

 
116.42

 
Vested
(103.2
)
 
89.44

 
Forfeited
(2.9
)
 
115.93

 
Unvested as of March 31, 2017
785.7

 
$
117.62

1.82

We determine the fair value of restricted stock units based on the closing market price of the Company's common stock on the grant date. As of March 31, 2018, we had $33.6 million of unamortized deferred compensation related to unvested restricted stock units, which we anticipate recognizing over varying periods into 2022.