Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities
Net loss	$ (200,698)	$ (103,086)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	25,289	12,927
Premium on marketable securities	(49)	(663)
Amortization of premium on marketable securities	(57)	87
Depreciation	396	199
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	208	(1,680)
Accounts payable	221	116
Accrued expenses and other liabilities	14,312	7,198
Net cash used in operating activities	(160,378)	(84,902)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	214,436	7,999
Purchases of marketable securities	(93,816)	(118,650)
Purchases of property and equipment	(556)	(901)
Increase in restricted cash	(285)	(525)
Net cash provided by (used in) investing activities	119,779	(112,077)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	6,998	728
Payments of offering costs		(599)
Proceeds from public offerings of common stock, net of commissions and underwriting discounts		330,175
Net cash provided by financing activities	6,998	330,304
Net increase (decrease) in cash and cash equivalents	(33,601)	133,325
Cash and cash equivalents at beginning of period	168,517	186,753
Cash and cash equivalents at end of period	134,916	320,078
Supplemental disclosure of non-cash financing activities
Purchases of property and equipment financed with landlord tenant incentive	$ 353
Purchases of property and equipment included in accounts payable or accrued expenses		$ 106