Capital Stock - Narrative (Details) $ / shares in Units, $ in Thousands			1 Months Ended		12 Months Ended		22 Months Ended
	Mar. 31, 2017	Feb. 13, 2017 USD ($) $ / shares shares	Aug. 31, 2018 USD ($) $ / shares shares	Feb. 28, 2018 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Class of Stock [Line Items]
Shares authorized (in shares) | shares					105,000,000		105,000,000
Common stock, shares authorized (in shares) | shares					100,000,000	100,000,000	100,000,000
Preferred stock, shares authorized (in shares) | shares					5,000,000		5,000,000
Common stock, par value (in dollars per share) | $ / shares					$ 0.01	$ 0.01	$ 0.01
Preferred stock, par value (in usd per share) | $ / shares					$ 0.01		$ 0.01
Payments of stock issuance costs		$ 1,500
Underwriter Option
Class of Stock [Line Items]
Proceeds from the sale of common stock				$ 37,900
Shares sold (in shares) | shares				7,414,996
Underwriting Agreement
Class of Stock [Line Items]
Sale price (in usd per share) | $ / shares				$ 5.50
ATM Agreement
Class of Stock [Line Items]
Proceeds from the sale of common stock					$ 2,747	$ 7,862
Payments of stock issuance costs					82	$ 330
Commission fee percent	3.00%
Maximum | ATM Agreement
Class of Stock [Line Items]
Aggregate offering					50,000
Pre-Merger
Class of Stock [Line Items]
Issuance of common stock in public offering, net of issuance cost (in shares) | shares		9,045,126
Sale price (in usd per share) | $ / shares		$ 4.50
Proceeds from the sale of common stock		$ 39,200
Post-Merger
Class of Stock [Line Items]
Issuance of common stock in public offering, net of issuance cost (in shares) | shares		6,359,628
Sale price (in usd per share) | $ / shares		$ 6.40
LLS
Class of Stock [Line Items]
Transaction value			$ 5,000
Sale price (in usd per share) | $ / shares			$ 6.62
Proceeds from the sale of common stock			$ 900
Shares sold (in shares) | shares			150,987
Common stock
Class of Stock [Line Items]
Proceeds from the sale of common stock					$ 2,700		$ 10,200
Shares sold (in shares) | shares					372,852		1,213,386
Weighted-average share price (in usd per share) | $ / shares					$ 7.37		$ 8.74
Signal | Signal Shares issued to Miragen Common Shareholders
Class of Stock [Line Items]
Exchange rate of common stock		0.7031