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	$ (82,947)	$ (51,983)	$ (28,507)
Reconciliation of net loss to net cash used in operating activities:
Change in fair value of warrant liability	0	(267)	(101)
Depreciation and amortization	4,424	2,591	1,666
Loss (Gain) on disposal of property and equipment	534	(32)	(28)
Amortization of net premiums (discounts) on marketable securities	415	507	(18)
Deferred rent	(552)	897	660
Non-cash stock-based compensation	6,090	2,480	2,091
Realized gains on marketable securities	(615)	0	(879)
Other	60	(200)	0
Changes in operating assets and liabilities:
Restricted cash	1,120	(431)	(986)
Accounts receivable - billed	(5,705)	(451)	954
Accounts receivable - unbilled	1,888	(3,418)	266
Prepaid expenses and other assets	(5,904)	402	40
Accounts payable and accrued expenses	13,979	4,184	2,009
Deferred revenue	(253)	(341)	258
Lease incentives received	452	703	4,346
Net cash used in operating activities	(67,014)	(45,359)	(18,229)
Investing Activities:
Capital expenditures	(7,268)	(5,785)	(4,341)
Proceeds from disposal of property and equipment	39	116	324
Net cash received from acquisition	0	3,034	0
Purchases of marketable securities	(176,469)	(14,754)	(48,652)
Proceeds from sales, maturities and redemptions of marketable securities	53,865	33,781	20,407
Net cash provided by (used in) investing activities	(129,833)	16,392	(32,262)
Financing Activities:
Principal payments on capital leases	(124)	(87)	(104)
Principal payments on notes payable	(671)	(473)	(60)
Proceeds from notes payable	0	1,450	650
Changes in restricted cash	(2)	(1)	(756)
Cash paid with acquisition	(171)	0	0
Net proceeds from sales of common stock, net of offering costs of $7.1 million, $6.1 million and $0.4 million, respectively	107,896	128,648	54,002
Proceeds from the exercise of stock options and employee stock purchases	2,789	1,498	54
Net cash provided by financing activities	109,717	131,035	53,786
Effect of exchange rate on cash and cash equivalents	(6)	4	0
Net increase (decrease) in cash and cash equivalents	(87,136)	102,072	3,295
Cash and cash equivalents at beginning of year	119,471	17,399	14,104
Cash and cash equivalents at end of year	32,335	119,471	17,399
Supplemental disclosure of non-cash activities:
Common stock issued in connection with acquisition	0	41,942	0
Capital expenditures included in accounts payable and accrued expenses	2,615	379	1,321
Deposit applied towards the purchase of equipment	0	0	500
Equipment acquired under a capital lease	0	0	399
Supplemental disclosure of cash flow information:
Cash interest payments	$ 179	$ 177	$ 20