Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Operating activities
Net loss	$ (139,611)	$ (115,118)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	463	1,001
Depreciation and amortization	6,281	4,747
Stock-based compensation expense	24,971	20,926
Other non-cash items	2,334	1,563
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(9,594)	(1,722)
Accounts payable	2,003	(3,132)
Accrued expenses and other liabilities	(8,509)	2,497
Deferred revenue	(7,682)	7,669
Deferred rent	507	469
Net cash used in operating activities	(128,837)	(81,100)
Investing activities
Purchase of property and equipment	(39,509)	(6,057)
Purchases of marketable securities	(116,740)	(80,845)
Proceeds from maturities of marketable securities	210,810	226,825
Restricted cash		209
Net cash provided by investing activities	54,561	140,132
Financing activities
Proceeds from public offering of common stock, net of issuance costs	436,805
Cash paid for contingent purchase price consideration		(2,025)
Reimbursement of tenant improvements for financing lease obligation	36,399
Payments on financing lease obligation	(120)
Proceeds from issuance of common stock	8,639	1,353
Net cash provided by (used in) financing activities	481,723	(672)
Increase in cash and cash equivalents	407,447	58,360
Cash and cash equivalents at beginning of period	278,887	164,269
Cash and cash equivalents at end of period	686,334	222,629
Non-cash investing and financing activities:
Assets acquired under financing lease obligation	2,297	24,725
Purchases of property and equipment included in accounts payable and accrued expenses	1,194	$ 909
Offering expenses included in accounts payable and accrued expenses	191
Tenant improvements under financing lease included in tenant improvements receivable	$ 3,635