Debt - Carrying amount and Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Carrying Amount
Convertible Notes
Convertible senior notes	$ 24,872	$ 24,001
Fair Value
Convertible Notes
Convertible senior notes	$ 36,916	49,560
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	$ 7,648	7,393
Convertible Senior Notes 0.375 Percent Due 2018 | Fair Value
Convertible Notes
Convertible senior notes	$ 10,271	14,129
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	$ 17,224	16,608
Convertible Senior Notes 1.25 Percent Due 2020 | Fair Value
Convertible Notes
Convertible senior notes	$ 26,645	$ 35,431