Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (98,942)	$ (89,380)	$ (91,579)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Stock-based compensation expense	16,641	17,362	16,857
Depreciation and amortization expense	7,862	7,259	4,205
Non-cash interest expense		3
Accretion of discount on investments	(214)	(216)	(386)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,056)	31	(2,598)
Deferred revenue	13,476	(11,888)	108,814
Accounts payable	(353)	(856)	3,581
Accrued expenses and other liabilities	732	2,162	5,027
Net cash provided by (used in) operating activities	(62,854)	(75,523)	43,921
Cash flows from investing activities:
Purchases of property and equipment	(1,937)	(4,676)	(21,492)
Purchases of investments	(21,832)	(97,934)	(290,594)
Sales and maturities of investments	136,087	158,312	246,494
Net cash provided by (used in) investing activities	112,318	55,702	(65,592)
Cash flows from financing activities:
Proceeds from exercise of stock options	147	116	2,138
Proceeds from issuance of common stock and restricted common stock	61
Payments for repurchase of common stock for employee tax withholdings	(197)	(33)
Issuance of common stock under ESPP plan	257
Net cash provided by financing activities	268	83	2,138
Net increase (decrease) in cash, cash equivalents and restricted cash	49,732	(19,738)	(19,533)
Cash, cash equivalents and restricted cash at beginning of year	36,201	55,939	75,472
Cash, cash equivalents and restricted cash at end of year	85,933	36,201	55,939
Supplemental disclosure of cash flow information:
Cash paid for interest		1	109
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued expenses	$ 157	$ 877	$ 1,650