Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
CASH FLOWS USED IN OPERATING ACTIVITIES:
Net loss	$ (83,898)	$ (40,022)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	216	78
Amortization of premium (accretion of discount) marketable securities, net	782	682
Stock-based compensation	10,632	5,850
Non-cash interest	0	156
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,772	(1,006)
Other long-term assets	(188)	(30)
Accounts payable	(3,201)	999
Accrued expenses and other current liabilities	1,248	(5,906)
Net cash used in operating activities	(72,637)	(39,199)
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of property and equipment	(919)	(186)
Purchases of marketable securities	(225,497)	(179,338)
Sales and maturities of marketable securities	258,257	75,802
Net cash used in investing activities	31,841	(103,722)
CASH FLOWS PROVIDED BY FINANCING ACTIVITIES:
Proceeds from exercise of stock options	1,672	359
Proceeds from issuance of common stock, net	0	158,414
Net cash provided by financing activities	1,672	158,773
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	(39,124)	15,852
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	159,678	28,518
CASH AND CASH EQUIVALENTS AT END OF PERIOD	120,554	44,370
SUPPLEMENTAL DISCLOSURES:
Cash paid for interest	0	1,253
Capital Expenditures Incurred but Not yet Paid	$ 345	$ 0