Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating activities
Net loss	$ (68,712)	$ (56,274)
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of contingent consideration	1,433	1,013
Depreciation and amortization	2,844	2,283
Stock-based compensation expense	11,481	10,143
Other non-cash items	838	868
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(3,105)	(2,271)
Accounts payable	(3,257)	(2,195)
Accrued expenses and other liabilities	(13,318)	211
Deferred revenue	(3,969)	9,222
Deferred rent	462	234
Net cash used in operating activities	(75,303)	(36,766)
Investing activities
Purchase of property and equipment	(26,024)	(3,894)
Purchases of marketable securities	(82,940)	(28,175)
Proceeds from maturities of marketable securities	104,510	96,515
Net cash (used in) provided by investing activities	(4,454)	64,446
Financing activities
Reimbursement of tenant improvements for financing lease obligation	13,831
Proceeds from issuance of common stock	3,040	639
Net cash provided by financing activities	16,871	639
(Decrease) increase in cash and cash equivalents	(62,886)	28,319
Cash and cash equivalents at beginning of period	278,887	164,269
Cash and cash equivalents at end of period	216,001	192,588
Non-cash investing and financing activities:
Assets acquired under financing lease obligation	1,269	12,513
Purchases of property and equipment included in accounts payable and accrued expenses	5,971	$ 1,086
Tenant improvements under financing lease included in tenant improvements receivable	$ 12,687