Debt - Components of convertible notes (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017		Jun. 30, 2017
Convertible Notes
Less current portion	$ 7,554	$ 7,393	[1]
Convertible senior notes noncurrent	17,016	16,608	[1]
Carrying Amount
Convertible Notes
Convertible senior notes, total	24,570	24,001
Less current portion	7,554	7,393
Convertible senior notes noncurrent	$ 17,016	16,608
Convertible Senior Notes 0.375 Percent Due 2018
Convertible Notes
Interest rate of debt (as a percent)	0.375%			0.375%
Convertible Senior Notes 0.375 Percent Due 2018 | Carrying Amount
Convertible Notes
Convertible senior notes, total	$ 7,554	7,393
Convertible Senior Notes 1.25 Percent Due 2020
Convertible Notes
Interest rate of debt (as a percent)	1.25%			1.25%
Convertible Senior Notes 1.25 Percent Due 2020 | Carrying Amount
Convertible Notes
Convertible senior notes, total	$ 17,016	$ 16,608

[1] The condensed consolidated balance sheet at December 31, 2017 has been derived from the audited financial statements at that date.