Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended					3 Months Ended		9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Apr. 30, 2018	Mar. 31, 2018	Jan. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Common Shares [Line Items]
Issuance price per share (in dollars per share)		$ 45.00
Proceeds from issuance of common shares		$ 49,889						$ 55,000	$ 179,996
Other offering expenses		$ 111						$ 2,987	$ 5,039
Issuance of common shares as payment for license agreement						$ 4,080	$ 0
Private Placement
Common Shares [Line Items]
Common stock sold (shares)				2,000,000
Issuance price per share (in dollars per share)				$ 27.50
Net proceeds from private placement stock issuance				$ 52,000
Underwriting discounts and commissions				2,800
Other offering expenses				200
BMS Agreement
Common Shares [Line Items]
Upfront payment under the BMS Amendment				$ 50,000
AstraZeneca | License Agreement
Common Shares [Line Items]
Payment for license agreement	$ 3,000
Issuance of common shares as payment for license agreement (shares)	109,523
Issuance of common shares as payment for license agreement	$ 4,080
ALS Biopharma
Common Shares [Line Items]
Exercise of warrants, net settlement of shares (shares)			325,000		275,000
Number of shares issued for exercise of warrants (shares)			261,140		228,219
RPI Purchase Agreement
Common Shares [Line Items]
Common stock sold (shares)		1,111,111
Proceeds from issuance of common shares		$ 50,000