Description of the business - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accumulated deficit	$ (2,281,852)				$ (1,498,808)				$ (2,281,852)	$ (1,498,808)
Net loss	(223,347)	$ (206,033)	$ (195,782)	$ (164,446)	$ (149,023)	$ (145,480)	$ (145,996)	$ (115,126)	(789,608)	(555,625)	$ (335,643)
Net Cash Provided by (Used in) Operating Activities									(564,384)	$ (413,426)	$ (280,553)
Cash, cash equivalents and marketable securities	$ 1,240,000								$ 1,240,000