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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2018
Restricted stock [Member]  
Summary of Changes in Non-vested Shares of Restricted Stock Outstanding
A summary of changes in non-vested restricted shares outstanding for the three months ended March 31, 2018 is presented below. 
 
 
Number of
non-vested
shares
 
Weighted average
grant-date
fair value
Non-vested restricted shares outstanding at December 31, 2017
 
4,026,110

 
$
35.63

Granted
 
1,180,032

 
51.42

Vested
 
(975,625
)
 
47.32

Forfeited
 
(66,489
)
 
34.53

Non-vested restricted shares outstanding at March 31, 2018
 
4,164,028

 
$
37.38