Supplemental Balance Sheet Information - Prepaid expenses and other current assets (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Prepaid expenses and other current assets:
Prepaid research and development costs	$ 42,932	$ 21,156
Prepaid taxes	16,720	9,894
Interest receivable	2,791	1,557
Other	7,482	3,016
Total	$ 69,925	$ 35,623