Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating activities
Net loss	$ (263,507)	$ (166,783)	$ (48,709)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Change in fair value of contingent consideration	2,675	2,344	246
Depreciation and amortization	9,648	7,419	4,228
Stock-based compensation expense	39,756	41,120	10,763
Issuance of restricted common stock in exchange for consulting services to non-employees			168
Noncash benefit on release of tax valuation allowance			(11,797)
Other non-cash items	2,825	1,513	142
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(14,318)	(6,847)	307
Accounts payable	6,658	2,541	(2,249)
Accrued expenses and other liabilities	20,889	9,423	9,969
Deferred revenue	4,565	11,171	(24,871)
Deferred rent	1,162	(330)	2,110
Net cash used in operating activities	(189,647)	(98,429)	(59,693)
Investing activities
Restricted cash	(4,379)	(8,816)	209
Purchase of property and equipment	(15,995)	(7,055)	(8,708)
Payments made for tenant improvements for capital lease	(12,034)
Purchases of marketable securities	(348,225)	(755,175)	(174,981)
Proceeds from maturities of marketable securities	443,364	199,179	30,960
Acquisition of business, net of cash acquired			(4,673)
Net cash provided by (used in) investing activities	62,731	(571,867)	(157,193)
Financing activities
Cash paid for contingent purchase price consideration	(2,025)	(453)
Reimbursement of tenant improvements for capital lease	1,663
Proceeds from public offering of common stock, net of issuance costs	234,962	477,064	353,226
Proceeds from issuance of common stock	6,934	10,109	5,226
Net cash provided by financing activities	241,534	486,720	358,452
Increase (decrease) in cash and cash equivalents	114,618	(183,576)	141,566
Cash and cash equivalents at beginning of period	164,269	347,845	206,279
Cash and cash equivalents at end of period	278,887	164,269	347,845
Non-cash investing and financing activities:
Construction financing lease obligation	48,034	61,901
Purchases of property and equipment included in accounts payable and accrued expenses	6,363	$ 2,089	387
Tenant improvements under capital lease included in prepaid expenses and other assets	8,542
Offering expenses included in accounts payable and accrued expenses	$ 231		$ 183