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Convertible Senior Notes (Tables)
12 Months Ended
Dec. 31, 2017
Convertible Senior Notes
Convertible senior notes consist of the following (in thousands):
 
 
 
December 31, 2016
 
 
December 31, 2017
 
 
 
Principal/
 
 
Debt
 
 
   
 
Principal/
 
 
Debt
 
     
   
Fair Value
   
Issuance
   
Net
   
Fair Value
   
Issuance
   
Net
 
 
 
Amount
 
 
Costs
 
 
Amount
 
 
Amount
 
 
Costs
 
 
Amount
 
3.25% convertible senior notes (due 2020) *
 
$
 
 
$
 
 
$
 
 
$
22,469
 
 
$
 
 
$
22,469
 
4.25% convertible senior notes (due 2018)
 
 
93,865
 
   
1,098
 
 
 
92,767
 
 
 
21,178
 
   
103
 
   
21,075
 
4.875% convertible senior notes (due 2020)
 
 
113,000
 
 
 
2,760
 
 
 
110,240
 
 
 
113,000
 
 
 
1,972
 
 
 
111,028
 
Total convertible senior notes, net of debt issuance costs
 
$
206,865
 
 
$
3,858
 
 
$
203,007
 
 
$
156,647
 
 
$
2,075
 
 
$
154,572
 
   * The amount presented for the 3.25% 2020 convertible senior notes within the table represents the fair value as of December 31, 2017. The principal amount of these notes is $2
4.25% Convertible Senior Notes (due 2018)  
Components of Convertible Senior Notes
Key components of the 4.25% convertible senior notes due 2018 consist of the following (in thousands):
 
 
 
Years Ended December 31,
 
 
2015
 
2016
 
2017
Contractual interest expense on the coupon
 
$
4,250
 
 
$
4,191
 
 
$
2,184
 
Amortization of debt discount and debt issuance costs recognized as interest expense
 
 
836
 
 
 
1,172
 
 
 
844
 
 
 
$
5,086
 
 
$
5,363
 
 
$
3,028
 


 

3.25% Convertible Senior Notes (due 2020)  
Components of Convertible Senior Notes
Key components of the 3.25% convertible senior notes due 2020 consist of the following (in thousands):
 
 
 
Years Ended December 31,
 
 
2015
 
2016
 
2017
Contractual interest expense
 
$
 
 
$
 
 
$
103
 
Amortization of debt issuance costs recognized as interest expense
 
 
 
 
 
 
 
 
 
 
 
$
 
 
$
 
 
$
103
 
4.875% Convertible Senior Notes (due 2020)  
Components of Convertible Senior Notes
Key components of the 4.875% convertible senior notes due 2020 consist of the following (in thousands):
 
 
 
Years Ended December 31,
 
 
2015
 
2016
 
2017
Contractual interest expense
 
$
5,606
 
 
$
5,508
 
 
$
5,509
 
Amortization of debt issuance costs recognized as interest expense
 
 
811
 
 
 
804
 
 
 
789
 
 
 
$
6,417
 
 
$
6,312
 
 
$
6,298