Selected Quarterly Financial Information (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
General and administrative expense	$ 3,940		$ 3,758		$ 4,486		$ 4,947		$ 3,928		$ 4,687		$ 4,849		$ 5,228		$ 17,131	$ 18,692	$ 17,554
Operating loss	(3,940)		(3,758)		(4,486)		(4,947)		(3,928)		(4,687)		(4,849)		(5,228)		(17,131)	(18,692)	(17,554)
Other income (loss), net	(120)		(379)		(89)		249		64		(2,405)		659		0		(339)	(1,682)	217
Interest income	984		1,004		1,087		801		615		513		527		420		3,876	2,075	1,935
Interest expense	(2,667)		(2,643)		(2,112)		(1,198)		(1,169)		(1,161)		(1,155)		(1,149)		(8,620)	(4,634)	(4,523)
Equity income (loss)	(12,985)		(12,874)		(23,497)		(17,002)		(17,283)		(16,345)		43,794		(9,495)		(66,358)	671	(39,599)
Net loss before income taxes	(18,728)		(18,650)		(29,097)		(22,097)		(21,701)		(24,085)		38,976		(15,452)		(88,572)	(22,262)	(59,524)
Income tax benefit (expense)	0		0		0		0		0		0		0		0		0	0	0
Net loss	$ (18,728)		$ (18,650)		$ (29,097)		$ (22,097)		$ (21,701)		$ (24,085)		$ 38,976		$ (15,452)		$ (88,572)	$ (22,262)	$ (59,524)
Net loss per share
Basic (in dollars per share)	$ (0.91)	[1]	$ (0.91)	[1]	$ (1.43)	[1]	$ (1.08)	[1]	$ (1.07)	[1]	$ (1.18)	[1]	$ 1.92	[1]	$ (0.76)	[1]	$ (4.34)	$ (1.09)	$ (2.85)
Diluted (in dollars per share)	$ (0.91)	[1]	$ (0.91)	[1]	$ (1.43)	[1]	$ (1.08)	[1]	$ (1.07)	[1]	$ (1.18)	[1]	$ 1.70	[1]	$ (0.76)	[1]	$ (4.34)	$ (1.09)	$ (2.85)

[1] Per share amounts for the quarters have each been calculated separately. Accordingly, quarterly amounts may not add to the annual amounts because of differences in the average common shares outstanding during each period. Additionally, in regard to diluted per share amounts only, quarterly amounts may not add to the annual amounts because of the inclusion of the effect of potentially dilutive securities only in the periods in which such effect would have been dilutive, and because of the adjustments to net income (loss) for the dilutive effect of partner company common stock equivalents and convertible securities.