Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS USED IN OPERATING ACTIVITIES:
Net loss	$ (254,237)	$ (182,804)	$ (101,526)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,951	586	176
Amortization of premium (accretion of discount) marketable securities, net	(264)	791	1,714
Stock-based compensation expense	35,003	26,058	14,734
Amortization of debt discount and issuance costs	4,816	0	183
Loss on retirement of note payable	0	0	1,572
Changes in operating assets and liabilities:
Inventory	(4,366)	0	0
Trade receivables, net	(4,441)	0	0
Prepaid expenses	(3,689)	(342)	(1,157)
Other current assets	(1,362)	4,949	(3,757)
Other long-term assets	(256)	(291)	(108)
Accounts payable	(2,213)	(100)	3,936
Accrued expenses and other current liabilities	22,563	11,349	(2,870)
Other non-current liabilities	(190)	0	0
Net cash used in operating activities	(206,685)	(139,804)	(87,103)
CASH FLOWS (USED IN) PROVIDED BY INVESTING ACTIVITIES:
Purchases of property and equipment	(2,341)	(2,936)	(1,231)
Payment of milestone related to intangible asset	(8,712)	0	0
Purchases of marketable securities	(429,296)	(260,547)	(579,088)
Sales and maturities of marketable securities	191,363	499,603	340,497
Net cash (used in) provided by investing activities	(248,986)	236,120	(239,822)
CASH FLOWS PROVIDED BY FINANCING ACTIVITIES:
Proceeds from exercise of stock options	17,478	2,573	2,337
Proceeds from issuance of convertible notes	305,000	0	0
Proceeds from issuance of common stock, net	0	0	482,250
Deferred financing costs	(9,360)	0	0
Payments on note payable	0	0	(25,000)
Fee for early prepayment of note payable	0	0	(1,502)
Proceeds from employee stock purchase plan	2,550	0	0
Net cash provided by financing activities	315,668	2,573	458,085
NET (DECREASE) INCREASE IN CASH AND CASH EQUIVALENTS	(140,003)	98,889	131,160
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	258,567	159,678	28,518
CASH AND CASH EQUIVALENTS AT END OF YEAR	118,564	258,567	159,678
SUPPLEMENTAL DISCLOSURES:
Cash paid for interest	0	0	1,490
Property and equipment purchases in accrued expense	$ 352	$ 675	$ 0