0.5% Convertible Senior Notes and Capped Call - Schedule of Interest Expense (Details) - Convertible Debt - Convertible Senior Notes Due 2023 - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018
Debt Instrument [Line Items]
Amortization of debt discount		$ 16,135	$ 0
Amortization of issuance costs		1,269	0
Contractual interest expense		2,175	$ 0
Total interest expense	$ 0	$ 19,579

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

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