Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
CASH FLOWS USED IN OPERATING ACTIVITIES:
Net loss	$ (56,939)	$ (40,463)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	252	96
Amortization of premium (discount) on marketable securities, net	(45)	566
Stock-based compensation	9,071	4,192
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(4,233)	4,214
Other long-term assets	0	(78)
Accounts payable	599	(2,420)
Accrued expenses and other current liabilities	(1,350)	155
Other non-current liabilities	(119)	0
Net cash used in operating activities	(52,764)	(33,738)
CASH FLOWS USED IN INVESTING ACTIVITIES:
Purchases of property and equipment	(406)	(379)
Purchases of marketable securities	(72,045)	(157,922)
Sales and maturities of marketable securities	38,447	135,846
Net cash used in investing activities	(34,004)	(22,455)
CASH FLOWS PROVIDED BY FINANCING ACTIVITIES:
Proceeds from exercise of stock options	1,792	929
Proceeds from issuance of shares under employee stock purchase plan	1,030	0
Net cash provided by financing activities	2,822	929
NET DECREASE IN CASH AND CASH EQUIVALENTS	(83,946)	(55,264)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	258,567	159,678
CASH AND CASH EQUIVALENTS AT END OF PERIOD	174,621	104,414
SUPPLEMENTAL DISCLOSURES:
Cash paid for income taxes	11	0
Property and equipment purchases in accrued expenses at period end	$ 1,030	$ 0