CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Activities:
Net loss	$ (156,937)	$ (82,947)	$ (51,983)
Reconciliation of net loss to net cash used in operating activities:
Change in fair value of warrant liability	0	0	(267)
Depreciation and amortization	5,983	4,424	2,591
Loss (Gain) on disposal of property and equipment	681	534	(32)
Amortization of net premiums on marketable securities	1,217	415	507
Deferred rent	(721)	(552)	897
Non-cash stock-based compensation	13,431	6,090	2,480
Realized gains on marketable securities	0	(615)	0
Other	243	60	(200)
Changes in operating assets and liabilities:
Restricted cash	(36,204)	1,120	(431)
Accounts receivable - billed	6,250	(5,705)	(451)
Accounts receivable - unbilled	2,229	1,888	(3,418)
Prepaid expenses and other assets	(10,269)	(5,904)	402
Accounts payable and accrued expenses	9,075	13,979	4,184
Deferred revenue	36,140	(253)	(341)
Lease incentives received	2,792	452	703
Net cash used in operating activities	(126,090)	(67,014)	(45,359)
Investing Activities:
Capital expenditures	(18,286)	(7,268)	(5,785)
Proceeds from disposal of property and equipment	18	39	116
Net cash received from acquisition	0	0	3,034
Purchases of marketable securities	(228,521)	(176,469)	(14,754)
Proceeds from sales, maturities and redemptions of marketable securities	225,519	53,865	33,781
Net cash provided by (used in) investing activities	(21,270)	(129,833)	16,392
Financing Activities:
Principal payments of capital leases	(67)	(124)	(87)
Principal payments of notes payable	(600)	(671)	(473)
Proceeds from notes payable	0	0	1,450
Changes in restricted cash	(126)	(2)	(1)
Cash paid with acquisition	0	(171)	0
Net proceeds from sales of common stock, net of offering costs of $11.9 million, $7.1 million and $6.1 million, respectively	204,275	107,896	128,648
Proceeds from the exercise of stock options and employee stock purchases	4,801	2,789	1,498
Net cash provided by financing activities	208,283	109,717	131,035
Effect of exchange rate on cash and cash equivalents	(150)	(6)	4
Net increase (decrease) in cash and cash equivalents	60,773	(87,136)	102,072
Cash and cash equivalents at beginning of year	32,335	119,471	17,399
Cash and cash equivalents at end of year	93,108	32,335	119,471
Supplemental disclosure of non-cash activities:
Common stock issued in connection with acquisition	0	0	41,942
Capital expenditures included in accounts payable and accrued expenses	2,797	2,615	379
Supplemental disclosure of cash flow information:
Cash interest payments	$ 96	$ 179	$ 177