Quarterly Financial Data (Selected Quarterly Financial Data) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Revenue	$ 22,000	$ 29,000	$ 103,000	$ 106,000	$ 128,000	$ 143,000	$ 102,000	$ 102,000	$ 260,000	$ 475,000
Total operating expenses	11,886,000	3,436,000	3,692,000	4,624,000	3,068,000	2,571,000	2,694,000	2,369,000	23,638,000	10,702,000
Loss from operations	(11,864,000)	(3,407,000)	(3,589,000)	(4,518,000)	(2,940,000)	(2,428,000)	(2,592,000)	(2,267,000)	(23,378,000)	(10,227,000)
Other income (expense), net	1,808,000	1,032,000	(262,000)	1,406,000	636,000	7,867,000	13,361,000	(12,226,000)	3,984,000	9,638,000
Income tax benefit					73,000				556,000	73,000
Net (loss) income	(9,500,000)	(2,375,000)	(3,851,000)	(3,112,000)	(2,231,000)	5,439,000	10,769,000	(14,493,000)	(18,838,000)	(516,000)
Accretion of Series B redeemable convertible preferred stock			(133,000)	(1,725,000)	(949,000)	(7,163,000)	(1,828,000)	(338,000)	(1,858,000)	(10,278,000)
Excess of fair value of consideration transferred on conversion of Series B Preferred Stock		(1,214,000)	(2,366,000)						(3,580,000)
Net loss attributable to common stockholders	$ (9,500,000)	$ (3,589,000)	$ (6,350,000)	$ (4,837,000)	$ (3,180,000)	$ (1,724,000)	$ 8,941,000	$ (14,831,000)	$ (24,276,000)	$ (10,794,000)
Net loss per share of common stock - basic	$ (0.70)	$ (0.32)	$ (0.73)	$ (0.82)	$ (0.54)	$ (0.30)	$ 1.27	$ (3.49)
Net loss per share of common stock - diluted	$ (0.76)	$ (0.32)	$ (0.78)	$ (0.82)	$ (0.54)	$ (0.30)	$ (0.33)	$ (3.49)