Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net loss	$ (148,876)	$ (72,398)	$ (35,876)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of premium and discounts on investments	80	528	391
Depreciation of property and equipment	125	77	17
Stock-based compensation	19,422	7,960	6,154
Changes in operating assets and liabilities:
Decrease (increase) in prepaid expenses and other assets	(3,076)	224	(1,971)
Increase in accounts payable	2,935	792	30
Increase in accrued expenses and other current liabilities	8,454	11,499	1,073
Increase in deferred revenue	228,699
Net cash provided by (used in) operating activities	107,763	(51,318)	(30,182)
Investing activities:
Purchases of available-for-sale securities	(574,508)	(163,290)	(133,730)
Proceeds from maturing available-for-sale securities	201,136	137,038	116,646
Purchase of property and equipment	(789)	(237)	(93)
Net cash used in investing activities	(374,161)	(26,489)	(17,177)
Financing activities:
Proceeds from issuance of common stock, net	375,307	38,733	71,299
Proceeds from the exercise of stock options	2,740	1,273	307
Net cash provided by financing activities	378,047	40,006	71,606
Net increase (decrease) in cash and cash equivalents	111,649	(37,801)	24,247
Cash and cash equivalents-beginning of year	30,376	68,177	43,930
Cash and cash equivalents-end of year	$ 142,025	$ 30,376	68,177
Supplemental schedule of noncash financing activities:
Reclassification of share repurchase obligation			$ 37