CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 211,727	$ 84,098
Marketable securities	93,394	192,898
Accounts receivable	13,643	2,764
Prepaid expenses	3,151	3,483
Other current assets	383	704
Total current assets	322,298	283,947
Property, equipment and software, net	49,983	17,961
Marketable securities, non-current	0	7,986
Other assets	1,602	1,369
Total assets	373,883	311,263
Current liabilities:
Accounts payable	2,451	3,995
Accrued expenses	38,581	16,134
Deferred revenue	7,202	4,261
Deferred rent	1,048	1,319
Lease exit liability	298	1,593
Other current liabilities	175	0
Total current liabilities	49,755	27,302
Deferred revenue, net of current portion	13,637	10,045
Deferred rent, net of current portion	11,253	4,867
Lease exit liability, net of current portion	0	298
Total liabilities	74,645	42,512
Stockholders' equity:
Common stock, $0.01 par value – 125,000,000 shares authorized, 36,859,077 and 34,870,607 shares outstanding at December 31, 2017 and 2016, respectively	369	349
Additional paid-in capital	611,270	561,198
Accumulated other comprehensive loss	(61)	(82)
Accumulated deficit	(312,340)	(292,714)
Total stockholders' equity	299,238	268,751
Total liabilities and stockholders' equity	$ 373,883	$ 311,263

Carrying value as of the balance sheet date of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

Name:	us-gaap_AccountsPayableCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.