Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (161,690)	$ (156,783)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	29,679	26,421
(Accretion) amortization of (discount) premium on investment debt securities	(570)	533
Amortization of deferred financing costs	958	754
Depreciation and amortization	4,610	2,428
Gain on lease termination	(1,995)
Loss on the disposal of fixed assets	2,682	1,331
Accretion of debt discount	8,715	6,869
Unrealized loss on investments		249
Changes in operating assets:
Prepaid expenses and other current assets	(3,741)	(3,004)
Accounts receivable	(10,666)	(3,008)
Inventory	(578)	(2,333)
Security deposits	849	7,468
Other assets	(19,622)
Changes in operating liabilities:
Accounts payable, accrued expenses and other current liabilities	14,597	(15,171)
Operating lease liabilities	(3,359)
Long-term other liabilities	10,993	(865)
Interest payable	575
Deferred revenue	(810)	(1,210)
Net cash used in operating activities	(129,373)	(136,321)
Cash flows from investing activities:
Purchases of investment debt securities	(451,902)	(294,537)
Sales and maturities of investment debt securities	181,575	178,041
Purchases of equipment, leasehold improvements, and furniture and fixtures	(1,024)	(45)
Net cash used by investing activities	(271,351)	(116,541)
Cash flows from financing activities:
Proceeds from issuance of 2026 Convertible Notes, net of issuance costs	223,424	261,357
Proceeds from issuance of common stock, net of issuance costs	227,180
Proceeds from exercise of options, net	1,738	486
Payments of employee withholding taxes related to stock-based awards	(1,595)
Net cash provided by financing activities	450,747	261,843
Effect of exchange rate changes	230	(1,274)
Net increase (decrease) in cash and cash equivalents	50,253	7,707
Cash and cash equivalents - beginning of period	43,248	70,013
Cash and cash equivalents - end of period	$ 93,501	$ 77,720