Convertible Senior Notes - Schedule of Interest Expense Recognized (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Debt Instrument [Line Items]
Total	$ 8,168	$ 17,064	$ 17,064
2022 Notes
Debt Instrument [Line Items]
Amortization of debt issuance cost	394		376
Amortization of debt discount	$ 7,774		7,402
Effective interest rate of the liability component	4.75%
2018 Notes
Debt Instrument [Line Items]
Amortization of debt issuance cost	$ 0		498
Amortization of debt discount	$ 0		$ 8,788
Effective interest rate of the liability component	6.50%

Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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Amount of amortization expense attributable to debt issuance costs.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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