Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Condensed Consolidated Statements of Operations and Comprehensive Income (Loss)
Collaboration revenue	$ 11	$ 41,035
Operating expenses:
Research and development	12,219	15,143
General and administrative	4,936	4,443
Total operating expenses	17,155	19,586
Other income (expense):
Interest income	306	452
Interest expense	(88)
Total other income (expense), net	218	452
Net income (loss)	(16,926)	21,901
Other comprehensive income (loss):
Unrealized gain (loss) on marketable securities	(29)	8
Comprehensive income (loss)	(16,955)	21,909
Net loss attributable to common stockholders - basic	(16,926)	21,901
Net loss attributable to common stockholders - diluted	$ (16,926)	$ 21,901
Net income (loss) per share attributable to common stockholders — basic (in dollars per share)	$ (0.35)	$ 0.72
Net income (loss) per share attributable to common stockholders — diluted (in dollars per share)	$ (0.35)	$ 0.70
Weighted-average number of shares of common stock used in net income (loss) per share attributable to common stockholders — basic	47,988,630	30,299,650
Weighted-average number of shares of common stock used in net income (loss) per share attributable to common stockholders — diluted	47,988,630	31,461,696
Type of Revenue	mrsn:CollaborationMember	mrsn:CollaborationMember