Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net loss	$ (218,415)	$ (192,143)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	(1,460)	2,099
Depreciation and amortization	9,899	7,132
Stock-based compensation expense	38,953	30,831
Other non-cash items	2,545	2,166
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(12,865)	(3,857)
Accounts payable	(447)	1,210
Accrued expenses and other liabilities	(6,943)	24,099
Deferred revenue	3,084	6,117
Deferred rent	475	1,805
Net cash used in operating activities	(185,174)	(120,541)
Investing activities
Purchase of property and equipment	(43,408)	(15,005)
Purchases of marketable securities	(621,004)	(145,135)
Proceeds from maturities of marketable securities	320,190	356,684
Proceeds from sales of marketable securities		7,500
Release of restricted cash	628
Increase in restricted cash		(4,379)
Net cash (used in) provided by investing activities	(343,594)	199,665
Financing activities
Proceeds from public offering of common stock, net of issuance costs	436,805
Cash paid for contingent purchase price consideration	(1,074)	(2,025)
Reimbursement of tenant improvements for financing lease obligation	38,021
Payments on financing lease obligation	(315)
Proceeds from issuance of common stock	14,406	3,786
Net cash provided by financing activities	487,843	1,761
Increase (decrease) in cash and cash equivalents	(40,925)	80,885
Cash and cash equivalents at beginning of period	278,887	164,269
Cash and cash equivalents at end of period	237,962	245,154
Non-cash investing and financing activities:
Assets acquired under financing lease obligation	2,467
Tenant improvements under financing lease included in tenant improvements receivable	2,013
Purchases of property and equipment included in accounts payable and accrued expenses	1,740	2,479
Construction financing lease obligation		38,090
Stock option exercise proceeds receivable	$ 845	$ 374