CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (62,304)	$ (85,462)
Adjustments to reconcile net loss to net cash used in operating activities:
Non-cash share-based compensation expense	7,330	3,088
Non-cash interest expense on liability related to sale of future royalties	6,813	0
Change in fair value of warrant liability	0	1,182
Loss from equity method investment	900	728
Other non-cash items	133	17
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,876)	(5,270)
Other assets	(25)	0
Accounts payable	(2,542)	4,224
Accrued expenses	6,701	(27)
Other long-term liabilities	(1,999)	(13)
Net cash used in operating activities	(46,869)	(81,533)
Cash flows from investing activities:
Purchases of property and equipment	(1,038)	(357)
Purchase of equity method investment	0	(1,375)
Net cash used in investing activities	(1,038)	(1,732)
Cash flows from financing activities:
Proceeds from issuance of common shares	0	55,000
Proceeds from exercise of stock options	1,065	1,003
Net cash provided by financing activities	1,065	56,003
Net increase in cash	(46,842)	(27,262)
Cash at beginning of period	264,249	131,468
Cash at end of period	217,407	104,206
Supplemental disclosure of cash flow information:
Cash paid for interest	0	0
Cash paid for income taxes	408	80
Supplemental disclosure of non-cash investing and financing activities:
Offering costs included in accounts payable and accrued expenses	$ 0	$ 2,987