Selected Quarterly Financial Information (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	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, 2019	Dec. 31, 2018
Condensed Income Statements, Captions [Line Items]
General and administrative expense	$ 2,060	$ 2,262	$ 2,603	$ 3,057	$ 2,618	$ 3,516	$ 5,148	$ 5,589	$ 9,982	$ 16,871
Operating loss	(2,060)	(2,262)	(2,603)	(3,057)	(2,618)	(3,516)	(5,148)	(5,589)	(9,982)	(16,871)
Other income (loss), net	2,245	8,777	3,118	(1,885)	(193)	(1,078)	(2,452)	(1,435)	12,255	(5,158)
Interest income	174	234	763	873	624	718	666	798	2,044	2,806
Interest expense	0	(5,806)	(5,682)	(2,535)	(6,645)	(3,310)	(3,422)	(2,690)	(14,023)	(16,067)
Equity income (loss), net	(1,057)	(3,440)	40,497	28,267	(7,791)	39,246	(14,540)	2,746	64,267	19,661
Net income (loss) before income taxes	(698)	(2,497)	36,093	21,663	(16,623)	32,060	(24,896)	(6,170)	54,561	(15,629)
Income tax benefit (expense)	0	0	0	0	0	0	0	0	0	0
Net income (loss)	$ (698)	$ (2,497)	$ 36,093	$ 21,663	$ (16,623)	$ 32,060	$ (24,896)	$ (6,170)	$ 54,561	$ (15,629)
Net loss per share
Basic (in dollars per share)	$ (0.03)	$ (0.12)	$ 1.75	$ 1.05	$ (0.81)	$ 1.56	$ (1.21)	$ (0.30)	$ 2.64	$ (0.76)
Diluted (in dollars per share)	$ (0.03)	$ (0.12)	$ 1.75	$ 1.05	$ (0.81)	$ 1.56	$ (1.21)	$ (0.30)	$ 2.64	$ (0.76)
Equity Method Investment Income Adjustment [Member]
Condensed Income Statements, Captions [Line Items]
Equity income (loss), net	$ 1,400