CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Activities:
Net loss	$ (279,966)	$ (156,937)	$ (82,947)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	8,505	5,983	4,424
Loss on disposal of property and equipment	374	681	534
Amortization of net premiums on marketable securities	257	1,217	415
Amortization of debt issuance costs	1,305	0	0
Lease incentives received	1,963	2,792	452
Non-cash stock-based compensation	19,160	13,431	6,090
Realized gains on marketable securities	0	0	(615)
Other	406	243	60
Changes in operating assets and liabilities:
Restricted cash	3,301	(36,204)	1,120
Accounts receivable	2,072	8,479	(3,817)
Prepaid expenses and other assets	(3,191)	(10,269)	(5,904)
Accounts payable and accrued expenses	(4,808)	9,075	13,979
Deferred revenue	(6,057)	36,140	(253)
Other liabilities	1,212	(721)	(552)
Net cash used in operating activities	(255,467)	(126,090)	(67,014)
Investing Activities:
Capital expenditures	(18,213)	(18,286)	(7,268)
Proceeds from disposal of property and equipment	11	18	39
Purchases of marketable securities	(356,556)	(228,521)	(176,469)
Proceeds from sales, maturities and redemptions of marketable securities	402,775	225,519	53,865
Net cash provided by (used in) investing activities	28,017	(21,270)	(129,833)
Financing Activities:
Principal payments of capital leases	(71)	(67)	(124)
Principal payments of notes payable	(395)	(600)	(671)
Changes in restricted cash	(819)	(126)	(2)
Cash paid with acquisition	0	0	(171)
Proceeds from issuance of convertible notes	325,000	0	0
Payments of costs related to issuance of convertible notes	(9,966)	0	0
Payments for capped call transactions and costs	(38,521)	0	0
Net proceeds from sales of common stock	0	204,275	107,896
Proceeds from the exercise of stock options and employee stock purchases	3,802	4,801	2,789
Net cash provided by financing activities	279,030	208,283	109,717
Effect of exchange rate on cash and cash equivalents	(335)	(150)	(6)
Net increase (decrease) in cash and cash equivalents	51,245	60,773	(87,136)
Cash and cash equivalents at beginning of year	93,108	32,335	119,471
Cash and cash equivalents at end of year	144,353	93,108	32,335
Supplemental disclosure of non-cash activities:
Capital expenditures included in accounts payable and accrued expenses	697	2,797	2,615
Supplemental disclosure of cash flow information:
Cash interest payments	$ 6,189	$ 96	$ 179