Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Common Stock [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]	Total
Balance at Dec. 31, 2016	$ 45,818	$ (654)	$ (35,302)	$ 9,862
Balance, shares at Dec. 31, 2016	28,737,000
Net loss			(19,375)	(19,375)
Unrealized loss on BioTime shares held as available-for-sale securities		(527)		(527)
Stock-based compensation	$ 1,630			1,630
Stock-based compensation, shares
Issuance of common stock upon exercise of 2016 warrants	$ 7,774			7,774
Issuance of common stock upon exercise of 2016 warrants, shares	2,392,000
Exercise of stock options	$ 610			610
Exercise of stock options, shares	323,000
Issuance of warrants for inducement to exercise 2016 warrants	$ 4,074			4,074
Issuance of warrants for inducement to exercise 2016 warrants, shares
Issuance of warrants to Silicon Valley Bank	$ 62			62
Issuance of warrants to Silicon Valley Bank, shares
Transfer of realized loss on sale of BioTime shares		293		293
Balance at Dec. 31, 2017	$ 59,968	(888)	(54,677)	4,403
Balance, shares at Dec. 31, 2017	31,452,000
Net loss			(15,754)	(15,754)
Stock-based compensation	$ 1,479			1,479
Stock-based compensation, shares
Exercise of stock options	$ 58			58
Exercise of stock options, shares	20,000
Cumulative-effect adjustment for adoption of ASU 2016-01 on January 1, 2018		888	(888)
Sale of common shares and warrants	$ 13,592			13,592
Sale of common shares and warrants, shares	9,192,000
Financing costs paid to issue common shares and warrants	$ (355)			(355)
Financing costs paid to issue common shares and warrants, shares
Balance at Dec. 31, 2018	$ 74,742		$ (71,319)	$ 3,423
Balance, shares at Dec. 31, 2018	40,664,000