Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (164,446)	$ (115,126)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	296	534
Depreciation and amortization	3,783	4,020
Stock-based compensation expense	32,341	22,995
Unrealized loss on equity securities	3,085
Other non-cash items	(3,456)	2,403
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(12,793)	(13,486)
Operating lease right-of-use assets	5,374
Accounts payable	10,590	(737)
Accrued expenses and other liabilities	(21,514)	4,541
Operating lease liabilities	3,224
Deferred revenue	(8,672)	(13,966)
Collaboration research advancement	(1,966)
Net cash used in operating activities	(154,154)	(108,822)
Cash flows from investing activities:
Purchase of property, plant and equipment	(19,321)	(7,452)
Purchases of marketable securities	(381,735)	(402,413)
Proceeds from maturities of marketable securities	364,143	145,140
Net cash used in investing activities	(36,913)	(264,725)
Cash flows from financing activities:
Proceeds from public offering of common stock, net of issuance costs		48,701
Reimbursement of assets under financing lease obligation		3,098
Payments on financing lease obligation		(106)
Proceeds from exercise of stock options and ESPP contributions	10,223	19,984
Net cash provided by financing activities	10,223	71,677
Decrease in cash, cash equivalents and restricted cash	(180,844)	(301,870)
Cash, cash equivalents and restricted cash at beginning of period	417,099	772,268
Cash, cash equivalents and restricted cash at end of period	236,255	470,398
Supplemental cash flow disclosures from investing and financing activities:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	9,302	2,817
Assets acquired under operating lease obligation	5,500
Tenant improvements included in receivables and other current assets	8,009	14
Restricted cash included in receivables and other current assets	$ 100	$ 100
Restricted Cash, Current, Asset, Statement of Financial Position [Extensible List]	blue:ReceivablesAndOtherCurrentAssets	blue:ReceivablesAndOtherCurrentAssets
Restricted cash included in restricted cash and other non-current assets	$ 14,417	$ 13,763
Restricted Cash, Noncurrent, Asset, Statement of Financial Position [Extensible List]	us-gaap:OtherAssetsNoncurrent	us-gaap:OtherAssetsNoncurrent