Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
CASH FLOWS USED IN OPERATING ACTIVITIES:
Net loss	$ (130,084)	$ (68,286)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	371	126
Amortization of premium on marketable securities, net	797	1,024
Stock-based compensation	18,702	10,022
Non-cash interest	0	183
Loss on retirement of note payable	0	1,572
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	3,308	(2,426)
Other long-term assets	(291)	(108)
Accounts payable	(3,563)	2,218
Accrued expenses and other current liabilities	7,284	(4,391)
Other non-current liabilities	402	0
Net cash used in operating activities	(103,074)	(60,066)
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of property and equipment	(2,125)	(230)
Purchases of marketable securities	(225,497)	(420,734)
Sales and maturities of marketable securities	367,141	164,474
Net cash provided by (used in) investing activities	139,519	(256,490)
CASH FLOWS PROVIDED BY FINANCING ACTIVITIES:
Payments on note payable	0	(25,000)
Fee for early prepayment of note payable	0	(1,502)
Proceeds from exercise of stock options	2,442	958
Proceeds from issuance of common stock, net	0	482,254
Net cash provided by financing activities	2,442	456,710
NET INCREASE IN CASH AND CASH EQUIVALENTS	38,887	140,154
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	159,678	28,518
CASH AND CASH EQUIVALENTS AT END OF PERIOD	198,565	168,672
SUPPLEMENTAL DISCLOSURES:
Cash paid for interest	0	1,490
Property and equipment purchases in accrued expenses at period end	$ 406	$ 0