Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOWS USED IN OPERATING ACTIVITIES:
Net loss	$ (182,804)	$ (101,526)	$ (62,479)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	586	176	77
Amortization of premium (accretion of discount) marketable securities, net	791	1,714	429
Stock-based compensation expense	26,058	14,734	7,070
Research and development expense settled in stock	0	0	2,717
Change in fair value of other current assets, warrant liability and other liability	0	0	505
Non-cash interest	0	183	295
Loss on retirement of note payable	0	1,572	57
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	4,607	(4,914)	(1,639)
Other long-term assets	(291)	(108)	(105)
Accounts payable	(100)	3,936	1,991
Accrued expenses and other current liabilities	11,349	(2,870)	2,737
Net cash used in operating activities	(139,804)	(87,103)	(48,345)
CASH FLOWS (USED IN) PROVIDED BY INVESTING ACTIVITIES:
Purchases of property and equipment	(2,936)	(1,231)	(857)
Purchases of marketable securities	(260,547)	(579,088)	(97,678)
Sales and maturities of marketable securities	499,603	340,497	20,470
Net cash (used in) provided by investing activities	236,120	(239,822)	(78,065)
CASH FLOWS PROVIDED BY FINANCING ACTIVITIES:
Proceeds from exercise of stock options	2,573	2,337	170
Net proceeds from the issuance of preferred stock, net	0	0	27,368
Proceeds from note payable, net	0	0	24,555
Proceeds from issuance of common stock, net	0	482,250	103,804
Deferred financing costs	0	0	(116)
Payments on note payable	0	(25,000)	(13,156)
Fee for early prepayment of note payable	0	(1,502)	0
Net cash provided by financing activities	2,573	458,085	142,625
NET INCREASE IN CASH AND CASH EQUIVALENTS	98,889	131,160	16,215
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	159,678	28,518	12,303
CASH AND CASH EQUIVALENTS AT END OF YEAR	258,567	159,678	28,518
SUPPLEMENTAL DISCLOSURES:
Cash paid for interest	0	1,490	1,971
Property and equipment purchases in accrued expense at period end	675	0	0
NON-CASH FINANCING ACTIVITIES:
Accretion of dividends on preferred stock	0	0	9,000
Reclassification of preferred stock to common stock	0	0	298,063
Fair value of series A-6 convertible preferred stock issued as settlement of liability	0	0	10,109
Fair value of warrants issued	$ 0	$ 0	$ 1,552