CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating Activities:
Net loss	$ (43,854)	$ (77,252)
Reconciliation of net loss to net cash used in operating activities:
Depreciation and amortization	2,111	1,954
Loss on disposal of property and equipment	65	0
Amortization of net (discounts) premiums on marketable securities	(6)	146
Amortization of debt issuance costs	356	237
Lease incentives received	1,248	0
Non-cash stock-based compensation	4,213	4,961
Other	0	98
Changes in operating assets and liabilities:
Restricted cash	5,880	1,278
Prepaid expenses and other assets	1,093	637
Accounts payable and accrued expenses	(4,867)	1,482
Deferred revenue	(5,399)	(1,644)
Other liabilities	543	(1,728)
Net cash used in operating activities	(38,617)	(69,831)
Investing Activities:
Capital expenditures	(1,084)	(5,517)
Proceeds from maturities of marketable securities	86,130	82,657
Purchases of marketable securities	(110,330)	(246,580)
Net cash used in investing activities	(25,284)	(169,440)
Financing Activities:
Principal payments on capital lease	(18)	(19)
Principal payments on notes payable	0	(143)
Proceeds from issuance of convertible notes	0	325,000
Payments of costs related to issuance of convertible notes	0	(9,966)
Payments for capped call transactions and costs	0	(38,521)
Net proceeds from sales of common stock	14,585	0
Proceeds from the exercise of stock options and employee stock purchases	797	2,000
Net cash provided by financing activities	15,364	278,351
Effect of exchange rate on cash and cash equivalents	31	37
Net (decrease) increase in cash and cash equivalents	(48,506)	39,117
Cash and cash equivalents at beginning of period	144,353	93,108
Cash and cash equivalents at end of period	95,847	132,225
Supplemental disclosure of non-cash activities:
Sale of common stock under the Sales Agreement not settled at quarter-end	225	0
Property and equipment purchases included in accounts payable and accrued expenses	823	3,180
Supplemental disclosure of cash flow information:
Cash payments of interest	$ 6,094	$ 12