Summary of Significant Accounting Policies - Impact of Adopting ASC 606, Pro Forma Statement of Operations and Comprehensive Loss (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenues:
License and milestone fees	$ 672,000	$ 79,000	$ 13,533,000	$ 49,889,000
Non-cash royalty revenue related to the sale of future royalties	8,441,000	6,503,000	22,873,000	20,555,000
Research and development support	388,000	650,000	1,159,000	3,030,000
Clinical materials revenue	1,427,000	1,248,000	2,465,000	2,525,000
Total revenues	10,928,000	8,480,000	40,030,000	75,999,000
Operating Expenses:
Research and development	47,243,000	31,689,000	130,775,000	99,896,000
General and administrative	8,347,000	7,908,000	26,994,000	24,863,000
Restructuring charge	870,000		3,287,000	386,000
Total operating expenses	56,460,000	39,597,000	161,056,000	125,145,000
Loss from operations	(45,532,000)	(31,117,000)	(121,026,000)	(49,146,000)
Investment income, net	1,369,000	293,000	2,845,000	551,000
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(2,546,000)	(3,385,000)	(8,203,000)	(10,461,000)
Interest expense on convertible senior notes	(23,000)	(762,000)	(70,000)	(3,012,000)
Other (expense) income, net	(75,000)	480,000	(590,000)	1,365,000
Net loss	$ (46,807,000)	$ (56,682,000)	$ (127,044,000)	$ (82,894,000)	$ (96,012,000)
Basic and diluted net loss per common share (in dollar per share)	$ (0.32)	$ (0.61)	$ (0.92)	$ (0.93)
Pro forma as if previous accounting was in effect | ASU 2014-09
Revenues:
License and milestone fees	$ 80,000		$ 15,239,000
Non-cash royalty revenue related to the sale of future royalties	7,562,000		23,658,000
Research and development support	388,000		1,159,000
Clinical materials revenue	1,427,000		2,465,000
Total revenues	9,457,000		42,521,000
Operating Expenses:
Research and development	47,243,000		130,775,000
General and administrative	8,347,000		26,994,000
Restructuring charge	870,000		3,287,000
Total operating expenses	56,460,000		161,056,000
Loss from operations	(47,003,000)		(118,535,000)
Investment income, net	1,369,000		2,845,000
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(2,546,000)		(8,203,000)
Interest expense on convertible senior notes	(23,000)		(70,000)
Other (expense) income, net	(75,000)		(590,000)
Net loss	$ (48,278,000)		$ (124,553,000)
Basic and diluted net loss per common share (in dollar per share)	$ (0.33)		$ (0.91)
Adjustments due to new guidance | ASU 2014-09
Operating Expenses:
Potential milestone payment	$ 500,000		$ 5,000,000