Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
CASH FLOWS USED IN OPERATING ACTIVITIES:
Net loss	$ (125,377)	$ (83,898)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	695	216
Amortization of premium (discount) on marketable securities, net	(75)	782
Stock-based compensation	20,533	10,632
Changes in operating assets and liabilities:
Inventory	(1,636)	0
Trade receivables, net	(1,211)	0
Prepaid expenses and other current assets	(3,625)	1,772
Other long-term assets	(7)	(188)
Accounts payable	(1,732)	(3,201)
Accrued expenses and other current liabilities	(466)	1,248
Other non-current liabilities	(48)	0
Net cash used in operating activities	(112,949)	(72,637)
CASH FLOWS (USED IN) PROVIDED BY INVESTING ACTIVITIES:
Purchases of property and equipment	(1,131)	(919)
Payments for capitalized milestones	(8,712)	0
Purchases of marketable securities	(111,983)	(225,497)
Sales and maturities of marketable securities	106,264	258,257
Net cash (used in) provided by investing activities	(15,562)	31,841
CASH FLOWS PROVIDED BY FINANCING ACTIVITIES:
Proceeds from exercise of stock options	4,024	1,672
Proceeds from issuance of shares under employee stock purchase plan	1,030	0
Net cash provided by financing activities	5,054	1,672
NET DECREASE IN CASH AND CASH EQUIVALENTS	(123,457)	(39,124)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	258,567	159,678
CASH AND CASH EQUIVALENTS AT END OF PERIOD	135,110	120,554
SUPPLEMENTAL DISCLOSURES:
Cash paid for income taxes	21	0
Property and equipment purchases in accrued expenses at period end	$ 1,247	$ 345