Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net loss	$ (163,406)	$ (74,598)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	44,116	15,817
Premium on marketable securities	(739)	(4)
Amortization of discount on marketable securities	(2,322)	(1,348)
Depreciation	415	247
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,637	(5,348)
Other long-term assets	(1,378)
Right of use operating asset	2,942
Operating lease liabilities	(2,500)
Accounts payable	(20,507)	(877)
Accrued expenses and other liabilities	(8,192)	(12,460)
Net cash used in operating activities	(149,934)	(78,571)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	307,263	81,960
Purchases of marketable securities	(580,916)	(394,109)
Purchases of property and equipment	(1,257)	(652)
Net cash used in investing activities	(274,910)	(312,801)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	15,681	12,757
Payment of employee tax obligations related to vesting of restricted stock units	(692)	(904)
Payments of offering costs	(70)	(235)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts	561,277	631,494
Net cash provided by financing activities	576,196	643,112
Net increase in cash, cash equivalents and restricted cash	151,352	251,740
Cash, cash equivalents and restricted cash at beginning of period	193,310	307,084
Cash, cash equivalents and restricted cash at end of period	344,662	558,824
Supplemental disclosure of non-cash investing and financing activities
Purchases of property and equipment included in accounts payable	78	15
Public offering costs included in accrued expenses	$ 259	$ 105