Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net loss	$ (115,126)	$ (68,712)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	534	1,433
Depreciation and amortization	4,020	2,844
Stock-based compensation expense	22,995	11,481
Other non-cash items	2,403	838
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(13,486)	(3,105)
Accounts payable	(737)	(3,257)
Accrued expenses and other liabilities	4,573	(13,318)
Deferred revenue	(13,966)	(3,969)
Deferred rent	(32)	462
Net cash used in operating activities	(108,822)	(75,303)
Cash flows from investing activities:
Purchase of property, plant and equipment, including assets under financing lease obligation	(7,452)	(26,024)
Purchases of marketable securities	(402,413)	(82,940)
Proceeds from maturities of marketable securities	145,140	104,510
Net cash used in investing activities	(264,725)	(4,454)
Cash flows from financing activities:
Reimbursement of assets under financing lease obligation	3,098	13,831
Payments on financing lease obligation	(106)
Proceeds from public offering of common stock, net of issuance costs	48,701
Proceeds from issuance of common stock	19,984	3,040
Net cash provided by financing activities	71,677	16,871
Decrease in cash, cash equivalents and restricted cash	(301,870)	(62,886)
Cash, cash equivalents and restricted cash at beginning of period	772,268	293,277
Cash, cash equivalents and restricted cash at end of period	470,398	230,391
Supplemental cash flow disclosures from investing and financing activities:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	2,817	5,971
Assets acquired under financing lease obligation		1,269
Tenant improvements under financing lease included in tenant improvements receivable	$ 14	$ 12,687