CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ 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
Income Statement [Abstract]
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 income (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)
Weighted average shares used in computing net income (loss) per share:
Basic (in shares)									20,636	20,544
Diluted (in shares)									20,636	20,544