Earnings (Loss) per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic and diluted loss per share
Net loss for the period	$ (30,824)	$ (52,825)	$ (17,788)	$ (13,342)	$ (22,262)	$ (22,269)	$ (14,602)	$ (15,223)	$ (114,780)	$ (74,356)	$ (54,890)
Weighted average number of shares outstanding									50,795,768	29,830,669	21,478,320
Basic and diluted loss per share	$ (0.46)	$ (0.90)	$ (0.44)	$ (0.36)	$ (0.61)	$ (0.79)	$ (0.54)		$ (2.26)	$ (2.49)	$ (2.56)