CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (37,565)	$ (30,229)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,226	222
Stock-based compensation expense	4,862	5,143
Change in fair value of contingent consideration		866
Changes in operating assets and liabilities:
Accounts receivable	(10,956)
Inventory	(65)
Prepaid expenses and other assets	1,854	1,022
Accounts payable	(197)	316
Accrued expenses	8,523	788
Net cash used in operating activities	(31,318)	(21,872)
Cash flows from investing activities:
Purchases of property and equipment	(284)	(298)
Purchases of marketable securities	(73,100)	(35,614)
Proceeds from sales and maturities of marketable securities	82,000	92,105
Net cash provided by investing activities	8,616	56,193
Cash flows from financing activities:
Finance lease payments	(120)	(36)
Proceeds from the exercise of employee stock options		394
Net cash (used in) provided by financing activities	(120)	358
Net increase (decrease) in cash and cash equivalents	(22,822)	34,679
Cash and cash equivalents at beginning of period	57,019	20,202
Cash and cash equivalents at end of period	34,197	54,881
Supplemental disclosures of non-cash investing and financing activities:
Additions to property and equipment included in accounts payable	24	210
Operating lease asset recorded as a result of new accounting standard	$ 2,132
Offering costs included in accounts payable		$ 20