Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 1,921	$ 2,196
Taxes receivable	0	16,737
Interest receivable on investments	414	969
Other current assets	0	43
Total prepaid expenses and other current assets	$ 2,335	$ 19,945