Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (406,602)	$ (218,415)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	843	(1,460)
Depreciation and amortization	12,613	9,899
Stock-based compensation expense	80,849	38,953
Other non-cash items	3,106	2,545
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(19,014)	(12,865)
Accounts payable	(493)	(447)
Accrued expenses and other liabilities	31,466	(6,468)
Deferred revenue	(31,981)	3,084
Collaboration research advancement	45,518
Net cash used in operating activities	(283,695)	(185,174)
Cash flows from investing activities:
Purchase of property, plant and equipment, including assets under financing lease obligation	(42,844)	(43,408)
Purchases of marketable securities	(827,163)	(621,004)
Proceeds from maturities of marketable securities	631,680	320,190
Purchase of other investment and investment receivable	(20,017)
Net cash used in investing activities	(258,344)	(344,222)
Cash flows from financing activities:
Proceeds from public offering of common stock, net of issuance costs	649,367	436,805
Cash paid for contingent purchase price consideration		(1,074)
Reimbursement of assets under financing lease obligation	3,098	38,021
Payments on financing lease obligation	(728)	(315)
Proceeds from issuance of common stock	29,550	14,406
Net cash provided by financing activities	735,771	487,843
Increase (decrease) in cash, cash equivalents and restricted cash	193,732	(41,553)
Cash, cash equivalents and restricted cash at beginning of period	772,268	293,277
Cash, cash equivalents and restricted cash at end of period	966,000	251,724
Supplemental cash flow disclosures from investing and financing activities:
Purchases of property, plant and equipment included in accounts payable and accrued expenses	2,770	1,740
Assets acquired under financing lease obligation		2,467
Stock option exercise proceeds receivable	49	845
Tenant improvements under financing lease included in tenant improvements receivable	14	$ 2,013
Regeneron Collaboration Agreement [Member]
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 54,484