Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (119,491,000)	$ (79,049,000)	$ (57,221,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	23,100,000	16,784,000	10,251,000
Amortization of investment premiums and discounts	732,000	2,582,000	3,465,000
Depreciation and amortization expense	956,000	383,000	48,000
Loss on foreign exchange	0	0	94,000
Write-off of property and equipment	0	0	21,000
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(784,000)	(742,000)	(767,000)
Other assets	2,000	6,000	(61,000)
Accounts payable	2,163,000	981,000	1,005,000
Accrued compensation	1,919,000	1,121,000	1,399,000
Accrued research and development expenses	1,598,000	(2,704,000)	4,288,000
Other current liabilities	1,896,000	215,000	293,000
Long-term liabilities	407,000	398,000	29,000
Net cash used in operating activities	(87,502,000)	(60,025,000)	(37,156,000)
Investing activities
Purchases of short-term investments	(176,459,000)	(304,928,000)	(379,776,000)
Sales of short-term investments	107,627,000	242,643,000	64,020,000
Maturities of short-term investments	188,973,000	149,046,000	96,113,000
Purchases of property and equipment	(20,232,000)	(3,020,000)	(290,000)
Restricted cash	(1,200,000)	0	(194,000)
Net cash provided by (used in) investing activities	98,709,000	83,741,000	(220,127,000)
Financing activities
Taxes paid related to net share settlement of restricted stock units	(357,000)	(94,000)	(4,647,000)
Proceeds from employee stock awards	1,249,000	600,000	439,000
Net cash provided by financing activities	20,048,000	506,000	259,226,000
Effect of exchange rates on cash	0	0	(94,000)
Increase in cash and cash equivalents	31,255,000	24,222,000	1,849,000
Cash and cash equivalents at beginning of period	47,968,000	23,746,000	21,897,000
Cash and cash equivalents at end of period	79,223,000	47,968,000	23,746,000
Non-cash investing and financing activities
Property and equipment purchases included in accounts payable and other accrued liabilities	10,122,000	352,000	0
Capitalized lease obligations	9,904,000	0	0
Property & equipment acquired under capital leases	1,076,000	0	0
Asset retirement cost	580,000	0	0
Interest capitalized during construction period for build-to-suit lease transaction	264,000	0	0
Proceeds from options exercised not yet received	439,000	0	0
Accrued costs related to underwritten public offering	160,000	0	0
Issuance of common stock upon vesting of stock awards	0	60,000	80,000
Change in long-term liabilities related to non-vested stock awards	0	(60,000)	(80,000)
Supplemental cash flow disclosure
Cash paid for taxes	0	10,000	3,000
Underwritten Offerings
Financing activities
Proceeds from sale of common stock, net	0	0	263,434,000
At The Market Offering
Financing activities
Proceeds from sale of common stock, net	$ 19,156,000	$ 0	$ 0