Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (74,356)	$ (54,890)	$ (46,964)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash other income, net	(1,371)	996	144
Non-cash interest income		(52)	(1)
Non-cash interest expense		35	21,465
Depreciation and amortization expense	432	233	153
Stock-based compensation	5,732	2,545	1,351
Deferred income taxes	1,410	(794)	(627)
Other, net	131	1	(5)
Changes in operating assets and liabilities:
Changes in long-term receivables	(5)	10	(71)
Changes in other receivables	(223)	(1,932)	(3,181)
Changes in accounts payable	2,585	(383)	2,651
Changes in accrued expenses and other liabilities	(3,778)	6,034	3,971
Changes in other non-current liabilities	96	24	9
Changes in income tax liabilities	(1)	(152)	(753)
Net cash used in operating activities	(69,348)	(48,325)	(21,858)
Cash flows from investing activities
Purchases of plant and equipment and intangible assets	(1,173)	(603)	(216)
Purchases of available-for-sale securities		(57,035)	(30,603)
Purchases of term deposits	(30)	(10)	(45,885)
Proceeds from sale of property, plant and equipment	2
Proceeds from maturities of term deposits		45,000
Proceeds from sales of available-for-sale securities	50,950	36,000
Net cash (used in) provided by investing activities	49,749	23,352	(76,704)
Cash flows from financing activities
Proceeds from initial public offering			106,088
Proceeds from April 2015 financing			44,836
Proceeds from December 2016 financing		24,822
Proceeds from September 2017 public offering	80,000
Other proceeds from shareholders			29
Proceeds from silent partnership			943
Proceeds from long-term borrowings	228
Proceeds from exercise of stock options	83	269	5
Proceeds from convertible loans			3,436
Repayments of participation rights			(6)
Repayments of long-term borrowings			(7,383)
Equity transaction costs	(8,092)	(2,790)	(14,930)
Net cash provided by financing activities	72,219	22,301	133,018
Effects of foreign currency translation on cash and cash equivalents	1,371	(996)	(160)
Net increase (decrease) in cash and cash equivalents	53,991	(3,668)	34,296
Cash and cash equivalents at beginning of year	32,778	36,446	2,150
Cash and cash equivalents at end of year	86,769	32,778	36,446
Supplemental disclosures of cash flow information:
Interest paid	(1)	(41)	(1,057)
Taxes paid	(5)	(867)	$ (939)
Equity transaction costs included in accrued expenses	$ 28	$ 1,451