Quarterly Financial Information (Unaudited) (Details) - USD ($)	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Total revenues	$ 44,864,000	$ 13,281,000	$ 15,542,000	$ 8,584,000	$ 13,416,000	$ 10,928,000	$ 9,287,000	$ 19,815,000	$ 39,448,000	$ 8,480,000	$ 39,020,000	$ 28,499,000	$ 82,271,000	$ 53,446,000	$ 115,447,000
Expenses:
Research and development	26,055,000	21,015,000	28,559,000	38,893,000	43,681,000	47,243,000	38,701,000	44,831,000	39,843,000	31,689,000	35,319,000	32,888,000	114,522,000	174,456,000	139,739,000
General and administrative	9,803,000	9,208,000	8,700,000	10,778,000	9,752,000	8,347,000	8,652,000	9,995,000	9,048,000	7,908,000	8,836,000	8,119,000	38,489,000	36,746,000	33,911,000
Restructuring charge	512,000	1,020,000	19,342,000	559,000	406,000	870,000	686,000	1,731,000	393,000			386,000	21,433,000	3,693,000	779,000
Total operating expenses	36,370,000	31,243,000	56,601,000	50,230,000	53,839,000	56,460,000	48,039,000	56,557,000	49,284,000	39,597,000	44,155,000	41,393,000	174,444,000	214,895,000	174,429,000
Loss from operations	8,494,000	(17,962,000)	(41,059,000)	(41,646,000)	(40,423,000)	(45,532,000)	(38,752,000)	(36,742,000)	(9,836,000)	(31,117,000)	(5,135,000)	(12,894,000)	(92,173,000)	(161,449,000)	(58,982,000)
Non-cash interest expense on liability related to sale of future royalty	(5,354,000)	(4,275,000)	(3,818,000)	(3,432,000)	(2,428,000)	(2,546,000)	(2,611,000)	(3,046,000)	(3,221,000)	(3,385,000)	(3,501,000)	(3,575,000)	(16,879,000)	(10,631,000)	(13,682,000)
Interest expense on convertible senior notes	(24,000)	(24,000)	(23,000)	(24,000)	(25,000)	(23,000)	(23,000)	(24,000)	(28,000)	(762,000)	(1,125,000)	(1,125,000)	(95,000)	(95,000)	(3,040,000)
Other income, net	1,698,000	511,000	1,454,000	1,351,000	1,077,000	1,294,000	(238,000)	1,199,000	691,000	773,000	894,000	249,000
Non-cash debt conversion expense									(724,000)	(22,191,000)					(22,915,000)
Net loss	$ 4,814,000	$ (21,750,000)	$ (43,446,000)	$ (43,751,000)	$ (41,799,000)	$ (46,807,000)	$ (41,624,000)	$ (38,613,000)	$ (13,118,000)	$ (56,682,000)	$ (8,867,000)	$ (17,345,000)	$ (104,133,000)	$ (168,843,000)	$ (96,012,000)
Basic and diluted net loss per common share (in dollars per share)	$ 0.03	$ (0.15)	$ (0.29)	$ (0.30)	$ (0.28)	$ (0.32)	$ (0.31)	$ (0.30)	$ (0.11)	$ (0.61)	$ (0.10)	$ (0.20)	$ (0.70)	$ (1.21)	$ (0.98)
License and milestone fees
Revenues:
Total revenues	$ 29,551,000	$ 79,000	$ 5,079,000	$ 79,000	$ 1,747,000	$ 672,000	$ 1,321,000	$ 11,540,000	$ 29,580,000	$ 79,000	$ 31,080,000	$ 18,730,000	$ 34,788,000	$ 15,280,000	$ 79,469,000
Non-cash royalty revenue related to the sale of future royalties
Revenues:
Total revenues	$ 15,313,000	$ 13,202,000	10,412,000	8,488,000	9,281,000	8,441,000	7,242,000	7,190,000	7,587,000	6,503,000	6,439,000	7,613,000	47,415,000	32,154,000	28,142,000
Research and development support
Revenues:
Total revenues			$ 51,000	$ 17,000	218,000	388,000	388,000	383,000	452,000	650,000	902,000	1,478,000	$ 68,000	1,377,000	3,482,000
Clinical materials revenue
Revenues:
Total revenues					$ 2,170,000	$ 1,427,000	$ 336,000	$ 702,000	$ 1,829,000	$ 1,248,000	$ 599,000	$ 678,000		$ 4,635,000	$ 4,354,000