Collaborative Arrangements and Revenue Recognition - Additional Information (Details) $ / shares in Units, $ in Millions	1 Months Ended			12 Months Ended
	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($) Installment shares	Mar. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 Milestone	Dec. 31, 2016 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of first warrant shares (in shares) | shares		4,000,000
Healios
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront cash payments received					$ 15.0
Potential payment for rights to additional indications including ARDS					$ 10.0
Common stock share issued in public offering (in shares) | shares			12,000,000
Shares of common stock issued, value			$ 21.1
Common stock issued, price per share (in dollars per share) | $ / shares			$ 1.76
Potential near-term payment received	$ 2.0	$ 10.0
Additional payment from Healios		$ 10.0
License and services revenue payment number of installments | Installment		4
License and services revenue payment installment amount due per quarter		$ 2.5
Expected credit to future milestone payment		10.0
Fair value of the warrant		$ 4.2	$ 5.3
Remaining exercisable warrants (in shares) | shares		16,000,000
Healios | Regulatory and Sales Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential revenue from approvals and milestones		$ 225.0
Number of future milestones achieved | Milestone				2
Healios | License
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Increase (decrease) in revenue		(1.1)
Ophthalmology License Agreement | Regulatory and Sales Milestones
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential revenue from approvals and milestones		$ 135.6