CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities:
Net loss	$ (184,748)	$ (183,769)	$ (279,966)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	8,159	9,817	8,505
(Gain) Loss on disposal of property and equipment	(55)	269	374
Non-cash impact of lease termination	(4,381)	0	0
Amortization of debt issuance costs	1,424	1,424	1,305
Lease incentives received	0	1,933	1,963
Non-cash stock-based compensation	18,314	19,809	19,160
Other	(2,396)	2,715	663
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,212	2,590	(1,119)
Accounts payable and accrued expenses	(6,744)	5,192	(4,808)
Deferred revenue	(15,610)	(4,456)	(6,057)
Other liabilities	0	0	1,212
Net cash used in operating activities	(184,825)	(144,476)	(258,768)
Investing Activities:
Capital expenditures	(1,372)	(4,189)	(18,202)
Purchases of marketable securities	(120,150)	(218,045)	(356,556)
Proceeds from maturities of marketable securities	150,118	258,202	402,775
Net cash provided by investing activities	28,596	35,968	28,017
Financing Activities:
Principal payments of capital leases	0	(37)	(71)
Principal payments of notes payable	0	0	(395)
Proceeds from issuance of convertible notes	0	0	325,000
Payments of costs related to issuance of convertible notes	0	0	(9,966)
Payments for capped call transactions and costs	0	0	(38,521)
Net proceeds from sales of common stock	100,060	63,425	0
Proceeds from the exercise of stock options and employee stock purchases	2,745	1,152	3,802
Net cash provided by financing activities	102,805	64,540	279,849
Effect of exchange rate on cash, cash equivalents and restricted cash	(48)	142	(335)
Net (decrease) increase in cash, cash equivalents and restricted cash	(53,472)	(43,826)	48,763
Cash, cash equivalents and restricted cash at beginning of year	135,431	179,257	130,494
Cash, cash equivalents and restricted cash at end of year	81,959	135,431	179,257
Supplemental disclosure of non-cash activities:
Capital expenditures included in accounts payable and accrued expenses	519	15	697
Supplemental disclosure of cash flow information:
Cash interest payments	$ 12,188	$ 12,188	$ 6,189