CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities:
Net loss	$ (183,769)	$ (279,966)	$ (156,937)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	9,817	8,505	5,983
Loss on disposal of property and equipment	269	374	681
Amortization of debt issuance costs	1,424	1,305	0
Lease incentives received	1,933	1,963	2,792
Non-cash stock-based compensation	19,809	19,160	13,431
Other	2,715	663	1,460
Changes in operating assets and liabilities:
Restricted cash	5,780	3,301	(36,204)
Prepaid expenses and other assets	2,590	(1,119)	(1,790)
Accounts payable and accrued expenses	5,192	(4,808)	9,075
Deferred revenue	(4,456)	(6,057)	36,140
Other liabilities	0	1,212	(721)
Net cash used in operating activities	(138,696)	(255,467)	(126,090)
Investing Activities:
Capital expenditures	(4,189)	(18,202)	(18,268)
Purchases of marketable securities	(218,045)	(356,556)	(228,521)
Proceeds from maturities of marketable securities	258,202	402,775	225,519
Net cash provided by (used in) investing activities	35,968	28,017	(21,270)
Financing Activities:
Principal payments of capital leases	(37)	(71)	(67)
Principal payments of notes payable	0	(395)	(600)
Changes in restricted cash	0	(819)	(126)
Proceeds from issuance of convertible notes	0	325,000	0
Payments of costs related to issuance of convertible notes	0	(9,966)	0
Payments for capped call transactions and costs	0	(38,521)	0
Net proceeds from sales of common stock	63,425	0	204,275
Proceeds from the exercise of stock options and employee stock purchases	1,152	3,802	4,801
Net cash provided by financing activities	64,540	279,030	208,283
Effect of exchange rate on cash and cash equivalents	142	(335)	(150)
Net (decrease) increase in cash and cash equivalents	(38,046)	51,245	60,773
Cash and cash equivalents at beginning of year	144,353	93,108	32,335
Cash and cash equivalents at end of year	106,307	144,353	93,108
Supplemental disclosure of non-cash activities:
Capital expenditures included in accounts payable and accrued expenses	15	697	2,797
Supplemental disclosure of cash flow information:
Cash interest payments	$ 12,188	$ 6,189	$ 96