Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (166,783)	$ (48,709)	$ (25,321)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Noncash benefit on release of tax valuation allowance		(11,797)
Depreciation and amortization	7,419	4,228	941
Stock-based compensation expense	41,120	10,763	6,491
Change in fair value of contingent consideration	2,344	246
Issuance of restricted common stock in exchange for consulting services to non-employees		168
Re-measurement of warrants			440
Other non-cash items	1,513	142	9
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(6,847)	307	(4,214)
Accounts payable	2,541	(2,249)	2,067
Accrued expenses and other liabilities	9,423	9,969	761
Deferred revenue	11,171	(24,871)	54,868
Deferred rent	(330)	2,110	7,408
Net cash (used in) provided by operating activities	(98,429)	(59,693)	43,450
Investing activities
Restricted cash	(8,816)	209	(1,153)
Purchase of property and equipment	(7,055)	(8,708)	(8,670)
Purchases of marketable securities	(755,175)	(174,981)
Proceeds from maturities of marketable securities	199,179	30,960
Acquisition of business, net of cash acquired		(4,673)
Net cash used in investing activities	(571,867)	(157,193)	(9,823)
Financing activities
Cash paid for contingent purchase price consideration	(453)
Proceeds from public offering of common stock, net of issuance costs	477,064	353,226	104,921
Repayment of nonrecourse note collateralized by restricted stock			344
Proceeds from issuance of common stock	10,109	5,226	376
Net cash provided by financing activities	486,720	358,452	105,641
(Decrease) increase in cash and cash equivalents	(183,576)	141,566	139,268
Cash and cash equivalents at beginning of period	347,845	206,279	67,011
Cash and cash equivalents at end of period	164,269	347,845	206,279
Non-cash investing and financing activities:
Construction financing lease obligation	61,901
Purchases of property and equipment included in accounts payable and accrued expenses	2,089	387	1,924
Offering expenses included in accounts payable and accrued expenses		183
Reclassification of warrants to additional paid-in capital			655
Conversion of preferred stock to common stock upon closing of IPO			122,178
Stock option exercise proceeds receivable	$ 17	$ 98	$ 107