Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Collaboration revenue	$ 828	$ 42,123	$ 10,594
Type of revenue	mrsn:CollaborationMember	mrsn:CollaborationMember	mrsn:CollaborationMember
Operating expenses:
Research and development	$ 67,036	$ 55,040	$ 59,915
General and administrative	21,902	17,283	16,334
Total operating expenses	88,938	72,323	76,249
Other income (expense):
Interest income	424	2,226	1,398
Interest expense	(359)	(234)	0
Total other income (expense), net	65	1,992	1,398
Net loss	(88,045)	(28,208)	(64,257)
Other comprehensive loss:
Unrealized gain (loss) on marketable securities	(25)	33	141
Comprehensive loss	(88,070)	(28,175)	(64,116)
Net loss attributable to common stockholders - basic	(88,045)	(28,208)	(64,257)
Net loss attributable to common stockholders - diluted	$ (88,045)	$ (28,208)	$ (64,257)
Net loss per share attributable to common stockholders — basic and diluted (in dollars per share)	$ (1.43)	$ (0.65)	$ (2.79)
Weighted-average number of common shares used in net loss per share attributable to common stockholders — basic and diluted (in shares)	61,485,205	43,492,113	23,032,250