Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (145,105)	$ (99,059)	$ (74,363)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation	1,410	970	252
Amortization of debt discounts	306	303	305
Amortization and accretion of premium or discount on investments, net	9	525	570
Acquisition of assets expensed to research and development	24,802	0	0
Stock-based compensation	26,078	16,794	12,945
Unrealized foreign exchange loss	601	0	0
Changes in operating assets and liabilities
Prepaid, current and other assets	(2,239)	(1,852)	(840)
Accounts payable, accrued expenses and other current liabilities	925	2,479	5,385
Net cash used in operating activities	(93,213)	(79,840)	(55,746)
Cash flows from investing activities
Acquisition of assets from Envisia	(10,500)	0	0
Purchase of available-for-sale investments	(104,490)	(35,169)	(46,872)
Proceeds from sales and maturities of investments	88,153	58,346	59,534
Purchase of property, plant and equipment	(15,970)	(5,077)	(3,280)
Net cash (used in) provided by investing activities	(42,807)	18,100	9,382
Cash flows from financing activities
Proceeds from sale of common stock, net	134,215	167,383	50,451
Proceeds related to issuance of stock for stock-based compensation arrangements, net	1,429	1,242	1,387
Net cash provided by financing activities	135,644	168,625	51,838
Net change in cash and cash equivalents	(376)	106,885	5,474
Cash and cash equivalents, at beginning of period	197,945	91,060	85,586
Cash and cash equivalents, at end of period	197,569	197,945	91,060
Supplemental disclosures
Cash paid for interest	2,188	2,192	2,186
Cash paid for income taxes	0	1,790	600
Non-cash investing and financing activities:
Capital lease obligation	689	0	0
Deferred costs from the sale of common stock	403	70	0
Build-to-suit lease transaction
Envisia Therapeutics Inc.
Non-cash investing and financing activities:
Equity issued for Envisia Asset Acquisition	$ 14,302	$ 0	$ 0