Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	$ 844	$ 2,151	$ 42,081	$ 9,405
Operating expenses:
Research and development	13,701	15,180	42,610	40,098
General and administrative	4,436	4,380	13,072	12,181
Total operating expenses	18,137	19,560	55,682	52,279
Other income (expense):
Interest income	608	340	1,785	1,049
Interest expense	(107)		(146)
Total other income (expense), net	501	340	1,639	1,049
Net loss	(16,792)	(17,069)	(11,962)	(41,825)
Other comprehensive loss:
Unrealized gain on marketable securities	17	48	36	107
Comprehensive loss	(16,775)	(17,021)	(11,926)	(41,718)
Net loss attributable to common stockholders - basic	(16,792)	(17,069)	(11,962)	(41,825)
Net loss attributable to common stockholders - diluted	$ (16,792)	$ (17,069)	$ (11,962)	$ (41,825)
Net loss per share attributable to common stockholders — basic and diluted (in dollars per share)	$ (0.35)	$ (0.74)	$ (0.28)	$ (1.82)
Weighted-average number of common shares used in net loss per share attributable to common stockholders — basic and diluted	47,833,607	23,152,019	42,011,340	22,979,516
Type of Revenue	mrsn:CollaborationMember	mrsn:CollaborationMember	mrsn:CollaborationMember	mrsn:CollaborationMember