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Debt (Tables)
 9 Months Ended
Sep. 30, 2018
Debt Disclosure [Abstract]  
Summary of convertible notes
The Convertible Notes consist of the following:
Liability component
 
September 30, 2018
 
December 31, 2017
Principal
 
$
150,000

 
$
150,000

Less: Debt issuance costs
 
(1,844
)
 
(2,121
)
Less: Debt discount, net(1)
 
(37,009
)
 
(42,572
)
Net carrying amount
 
$
111,147

 
$
105,307

 
(1) Included in the consolidated balance sheets within convertible senior notes (due 2022) and amortized to interest expense over the remaining life of the Convertible Notes using the effective interest rate method.
Summary of interest expense recognized related to the Convertible Notes
The following table sets forth total interest expense recognized related to the Convertible Notes:
 
 
Three Months Ended September 30,
 
Nine Months Ended September 30,
 
 
2018
 
2017
 
2018
 
2017
Contractual interest expense
 
$
1,134

 
$
1,134

 
$
3,375

 
$
3,375

Amortization of debt issuance costs
 
95

 
86

 
277

 
249

Amortization of debt discount
 
1,919

 
1,725

 
5,563

 
4,999

Total
 
$
3,148

 
$
2,945

 
$
9,215

 
$
8,623

Effective interest rate of the liability component
 
11
%
 
11
%
 
11
%
 
11
%