Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities
Net loss	$ (48,709)	$ (25,321)	$ (23,670)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Noncash benefit on release of income tax valuation allowance	(11,797)
Depreciation and amortization	4,228	941	301
Stock-based compensation expense	10,763	6,491	822
Remeasurement of contingent consideration	246
Issuance of restricted common stock in exchange for consulting services to non-employees	168
Re-measurement of warrants		440	(28)
Loss on disposal of equipment	3	9	10
Other non-cash items	139
Changes in operating assets and liabilities:
Prepaid expenses and other assets	307	(4,214)	93
Accounts payable	(2,249)	2,067	380
Accrued expenses and other liabilities	9,969	761	1,336
Deferred revenue	(24,871)	54,868	(340)
Deferred rent	2,110	7,408	52
Net cash (used in) provided by operating activities	(59,693)	43,450	(21,044)
Investing activities
Restricted cash	209	(1,153)	(40)
Purchase of property and equipment	(8,708)	(8,670)	(867)
Purchases of marketable securities	(174,981)
Proceeds from sales or maturities of marketable securities	30,960		3,506
Acquisition of business, net of cash acquired	(4,673)
Net cash (used in) provided by investing activities	(157,193)	(9,823)	2,599
Financing activities
Proceeds from public offering of common stock, net of issuance costs	353,226	104,921
Proceeds from issuance of convertible preferred stock, net of issuance costs			59,831
Repayment of nonrecourse note collateralized by restricted stock		344
Proceeds from exercise of stock options and issuance of common stock	5,226	376	21
Net cash provided by financing activities	358,452	105,641	59,852
Increase in cash and cash equivalents	141,566	139,268	41,407
Cash and cash equivalents at beginning of period	206,279	67,011	25,604
Cash and cash equivalents at end of period	347,845	206,279	67,011
Non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued expenses	387	1,924
Offering expenses included in accounts payable and accrued expenses	183
Accretion and dividends on convertible preferred stock			3,057
Gain on extinguishment of convertible preferred stock			23,114
Reclassification of warrants to additional paid-in capital		655	394
Reclassification of Series A-1 Preferred Stock to common stock			2,337
Conversion of preferred stock to common stock upon closing of IPO		122,178
Stock option exercise proceeds receivable	$ 98	$ 107