CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (117,144)	$ (102,052)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation expense	1,751	738
Stock-based compensation expense	16,505	16,281
Non-cash interest expense	2,259
Amortization of premium on investment securities	(1,094)	(225)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	2,596	(191)
Accounts payable	5,466	2,559
Accrued liabilities	(2,887)	6,366
Other liabilities	553	207
Net cash used in operating activities	(91,995)	(76,317)
Cash flows from investing activities:
Purchase of property and equipment	(4,860)	(6,243)
Purchase of investments	(106,980)	(133,208)
Maturities of investments	127,146	88,618
Net cash provided by (used in) investing activities	15,306	(50,833)
Cash flows from financing activities:
Borrowings under debt agreement	40,000
Debt issuance costs	(3,856)
Proceeds from underwritten public offering, net of offering costs		190,435
Net cash proceeds from exercise of stock options, including early exercise	6,270	5,385
Tax withholdings related to net share settlements of restricted stock units	(672)
Net cash provided by financing activities	41,742	195,820
Net increase (decrease) in cash and cash equivalents	(34,947)	68,670
Cash and cash equivalents at the beginning of the period	107,511	73,487
Cash and cash equivalents at the end of the period	72,564	142,157
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases included in accounts payable and accrued liabilities	1,916	$ 556
Debt issuance costs, discount and interest payable	$ 2,259