Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue:
Total revenues	$ 13,296	$ 7,851	$ 25,767	$ 23,808
Operating expenses:
Research and development	146,540	115,014	269,180	212,123
General and administrative	68,631	41,168	128,910	76,094
Cost of license and royalty revenue	$ 613	$ 21	$ 1,043	$ 36
Type of cost, good or service [Extensible List]	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember	blue:LicenseAndRoyaltyMember
Change in fair value of contingent consideration	$ 214	$ 262	$ 510	$ 796
Total operating expenses	215,998	156,465	399,643	289,049
Loss from operations	(202,702)	(148,614)	(373,876)	(265,241)
Interest income, net	9,387	2,436	19,489	3,824
Other (expense) income, net	(2,936)	182	(6,325)	297
Loss before income taxes	(196,251)	(145,996)	(360,712)	(261,120)
Income tax benefit	469		484
Net loss	$ (195,782)	$ (145,996)	$ (360,228)	$ (261,120)
Net loss per share - basic and diluted:	$ (3.55)	$ (2.91)	$ (6.54)	$ (5.22)
Weighted-average number of common shares used in computing net loss per share - basic and diluted:	55,165	50,153	55,062	50,038
Other comprehensive income (loss):
Other comprehensive income (loss), net of tax expense of $0.8 million and $0.0 million for the three months ended June 30, 2019 and 2018, respectively, and $1.3 million and $0.0 million for the six months ended June 30, 2019 and 2018, respectively	$ 973	$ (345)	$ 2,808	$ (1,189)
Total other comprehensive income (loss)	973	(345)	2,808	(1,189)
Comprehensive loss	(194,809)	(146,341)	(357,420)	(262,309)
Collaboration [Member]
Revenue:
Total revenues	11,558	7,437	22,735	23,045
License and Royalty [Member]
Revenue:
Total revenues	$ 1,738	$ 414	$ 3,032	$ 763