Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net loss	$ (99,059)	$ (74,363)	$ (48,133)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation	970	252	73
Amortization of deferred financing costs and debt discount	303	305	77
Amortization and accretion of premium or discount on available-for-sale investments, net	525	570	416
Stock-based compensation	16,794	12,945	9,178
Changes in operating assets and liabilities
Prepaid, current and other assets	(1,852)	(840)	(717)
Accounts payable and other current liabilities	2,479	5,385	4,829
Interest payable	0	0	551
Net cash used in operating activities	(79,840)	(55,746)	(33,726)
Cash flows from investing activities
Purchase of available-for-sale investments	(35,169)	(46,872)	(95,376)
Maturity of available-for-sale investments	53,156	55,785	20,739
Sale of available-for-sale investments	5,190	3,749	1,500
Purchase of furniture, fixtures and equipment	(5,077)	(3,280)	(181)
Net cash provided by (used in) investing activities	18,100	9,382	(73,318)
Cash flows from financing activities
Proceeds from sale of common stock, net of commissions and underwriting discounts	168,479	50,451	0
Payments of stock issuance costs and expenses	(1,096)	0	0
Proceeds from exercise of stock options	591	1,282	9
Proceeds from exercise of stock purchase rights	804	96	119
Tax withholdings related to restricted stock awards	(153)	0	0
Proceeds from exercise of warrants	0	9	0
Payments of debt issuance costs	0	0	122,853
Net cash provided by financing activities	168,625	51,838	122,981
Net change in cash and cash equivalents	106,885	5,474	15,937
Cash and cash equivalents
Beginning of period	91,060	85,586	69,649
End of period	197,945	91,060	85,586
Supplemental disclosures
Interest paid	2,192	2,186	0
Income taxes paid	1,790	600	0
Non-cash financing activities
Deferred costs from the sale of common stock and issuance of convertible notes	$ 70	$ 0	$ 25