Convertible Senior Notes and Interest Expense - Components of the Interest Expense Recognized (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt Disclosure [Abstract]
Stated coupon interest expense	$ 279		$ 558
Amortization of debt issuance costs	62	$ 0	124
Accretion of debt discount	545	$ 0	1,079
Total	$ 886		$ 1,761
Effective interest rate	8.40%		8.40%

Amount of the cost of borrowed funds accounted for as interest expense.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration