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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 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 six months ended June 30, 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
 
1,277,491

 
52.47

Vested
 
(1,041,748
)
 
47.10

Forfeited
 
(178,957
)
 
37.27

Non-vested restricted shares outstanding at June 30, 2018
 
4,082,896

 
$
37.90