Collaborative Arrangements and Revenue Recognition - Additional Information (Details)	1 Months Ended				12 Months Ended
	Dec. 31, 2018 USD ($) shares	Jun. 30, 2018 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	Feb. 28, 2017 shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) Installment shares	Dec. 31, 2017 USD ($) Milestone	Dec. 31, 2016 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Common stock share issued in public offering (in shares) | shares				22,772,300
Shares of common stock issued, value						$ 16,628,000	$ 39,813,000	$ 4,230,000
Transaction price expected to be recognized	$ 3,600,000					3,600,000
Number of first warrant shares (in shares) | shares		4,000,000
Healios License Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront cash payments received								15,000,000
Potential payment for rights to additional indications including ARDS								$ 10,000,000
Common stock share issued in public offering (in shares) | shares			12,000,000
Shares of common stock issued, value			$ 21,100,000			20,995,000
Common stock issued, price per share (in dollars per share) | $ / shares		$ 1.76	$ 1.76
Potential near-term payment received	2,000,000					10,000,000
Additional payment from Healios						$ 10,000,000
License and services revenue payment number of installments | Installment						4
License and services revenue payment installment amount due per quarter	$ 2,500,000					$ 2,500,000
Expected credit to future milestone payment						$ 10,000,000
Fair value of the warrant		$ 4,200,000	$ 5,300,000
Remaining exercisable warrants (in shares) | shares	16,000,000	16,000,000				16,000,000
Healios License Agreement | Regulatory and Sales Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential revenue from approvals and milestones						$ 225,000,000
Number of future milestones achieved | Milestone							2
Healios License Agreement | Forecast
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional payment from Healios					$ 3,000,000
Healios License Agreement | License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase (decrease) in revenue		$ 1,100,000
Ophthalmology License Agreement | Regulatory and Sales Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential revenue from approvals and milestones						135,600,000
RTI Surgical Inc
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Aggregated milestone revenue						$ 0	$ 1,000,000
Animal health company agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Nonrefundable option fee							$ 300,000