Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (9,923)	$ (12,956)	$ (11,321)	$ (10,155)	$ (44,582)	$ (24,283)	$ (32,241)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation					698	855	684
Stock-based compensation					4,851	3,849	3,044
Discount on revenue from issuance of warrant					0	1,080	0
Other					0	0	(22)
Stock–based patent license and settlement expense					0	315	3,150
Gain from insurance proceeds, net					0	(617)	0	$ (700)
Change in fair value of warrant liabilities					0	0	(728)
Changes in operating assets and liabilities:
Prepaid expenses and other					857	(1,346)	(206)
Net cash used in operating activities					(35,326)	(13,350)	(24,018)
Investing activities
Proceeds from insurance, net					0	617	0
Purchases of equipment					(579)	(1,532)	(285)
Net cash used in investing activities					(579)	(915)	(285)
Financing activities
Proceeds from exercise of warrants					0	0	1,862
Shares retained for withholding tax payments on stock-based awards					(424)	(402)	(283)
Net cash provided by financing activities					19,887	36,008	38,866
(Decrease) increase in cash and cash equivalents					(16,018)	21,743	14,563
Cash and cash equivalents at beginning of year		$ 51,059		$ 29,316	51,059	29,316	14,753
Cash and cash equivalents at end of year	$ 35,041		$ 51,059		35,041	51,059	29,316	$ 14,753
Consolidated entity, excluding related party
Changes in operating assets and liabilities:
Accounts receivable					245	324	12
Accounts payable and accrued expenses					(1,836)	4,269	1,537
Deferred revenue					0	(250)	771
Financing activities
Proceeds from issuance of common stock, net					20,311	15,415	37,287
Healios
Changes in operating assets and liabilities:
Accounts receivable					3,783	(4,545)	(153)
Accounts payable to Healios					1,068	0	0
Advances and deposits from Healios					199	4,889	134
Deferred revenue					(609)	2,110	0
Financing activities
Proceeds from issuance of common stock, net					$ 0	$ 20,995	$ 0