Net Loss per Share - Calculation of basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net loss	$ (39,269)	$ (38,440)	$ (124,731)	$ (57,193)
Accretion of beneficial conversion feature on Series A convertible preferred shares	0	(8,006)	0	(12,006)
Net loss attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.	$ (39,269)	$ (46,446)	$ (124,731)	$ (69,199)
Denominator:
Weighted average common shares outstanding - basic and diluted (in shares)	38,942,545	26,038,192	37,873,755	19,563,525
Net loss per share attributable to common shareholders of Biohaven Pharmaceutical Holding Company Ltd.-basic and diluted (in dollars per share)	$ (1.01)	$ (1.78)	$ (3.29)	$ (3.54)