Net loss per share (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	13,622,486	13,510,411
Stock Options
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	13,104,711	12,598,407
Convertible Senior Notes 0.375 Percent Due 2018
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	148,836	505,004
Convertible Senior Notes 1.25 Percent Due 2020
Anti-dilutive securities
Potential common shares excluded from diluted net loss per share computation	368,939	407,000