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Shareholders' Equity (Tables)
 12 Months Ended
Dec. 31, 2017
Equity [Abstract]  
Summary of Activities in Common Stock
The following table details activity in our common stock:
 
Years Ended December 31,
 
2017
 
2016
 
2015
 
Shares
 
Amount
 
Shares
 
Amount
 
Shares
 
Amount
Stock option exercises
1,113

 
$
49,845

 
827

 
$
23,527

 
1,118

 
$
23,160

Purchases of common stock
(44
)
 
(6,613
)
 
(882
)
 
(112,699
)
 
(5
)
 
(645
)
Employee stock plan purchases
51

 
7,044

 
47

 
6,236

 
43

 
4,671

Shares issued for acquisitions
—

 
—

 
—

 
—

 
2,149

 
364,333