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Debt and Other Financing Arrangements (Tables)
 12 Months Ended
Dec. 31, 2017
Debt Disclosure [Abstract]  
Schedule of the net carrying amount of debt
The net carrying amounts of the liability components of the 2018 and 2021 Notes as of December 31, 2017 and 2016 consists of the following (in thousands):
 
December 31, 2017
 
December 31, 2016
Principal amount
$
553,000

 
$
253,000

Unamortized debt discount
(10,190
)
 
(12,550
)
Net carrying amount before unamortized debt issuance costs
542,810

 
240,450

Unamortized debt issuance costs
(9,617
)
 
(2,015
)
Net carrying value
$
533,193

 
$
238,435
Schedule of note interest expense
The following table presents the interest expense recognized related to the 2018 Notes and the 2021 Notes for years ended December 31, 2017, 2016 and 2015 (in thousands):
 
Years Ended December 31,
 
2017
 
2016
 
2015
Contractual interest expense at 1.50% and 5.75% per annum
$
4,897

 
$
3,795

 
$
3,795

Amortization of debt issuance costs
1,472

 
1,263

 
1,202

Accretion of debt discount
8,360

 
7,867

 
7,489

Total
$
14,729

 
$
12,925

 
$
12,486