STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Operating Activities:
Net loss	$ (13,810)	$ (9,996)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	980	449
Amortization of net premiums on investments	99	94
Loss (gain) on disposal of property and equipment	3	(17)
Deferred rent	213	240
Non-cash stock-based compensation	1,040	483
Realized gains on investments	(615)
Changes in operating assets and liabilities:
Restricted cash	1,081	769
Accounts receivable	(2,559)	(1,591)
Unbilled receivables	(1,125)	(1)
Prepaid expenses and other assets	(1,516)	54
Accounts payable and accrued expenses	(4,007)	(1,590)
Deferred revenue	(215)	(180)
Lease incentives received		703
Net cash used in operating activities	(20,431)	(10,583)
Investing Activities:
Capital expenditures	(889)	(1,542)
Proceeds from disposal of property and equipment		51
Proceeds from maturities of investments	10,590	5,235
Purchases of investments		(8,718)
Net cash provided by (used in) investing activities	9,701	(4,974)
Financing Activities:
Principal payments on capital leases	(6)	(14)
Principal payments on notes payable	(168)	(39)
Proceeds from notes payable		809
Restricted cash	(1)	(1)
Net proceeds from sales of common stock, net of offering costs of $0.2 million		6,425
Proceeds from the exercise of stock options and employee stock purchases	1,345	1
Net cash provided by financing activities	1,170	7,181
Effect of exchange rate on cash and cash equivalents	10
Net decrease in cash and cash equivalents	(9,550)	(8,376)
Cash and cash equivalents at beginning of period	119,471	17,399
Cash and cash equivalents at end of period	109,921	9,023
Supplemental disclosure of non-cash activities:
Sale of common stock under the 2012 Sales Agreement not settled at quarter-end		376
Property and equipment purchases included in accounts payable and accrued expenses	214	422
Supplemental disclosure of cash flow information:
Cash payments of interest	$ 52	$ 20