Quarterly Data And Share Information (Schedule Of Quarterly Data) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Quarterly Data And Share Information [Abstract]
Revenues	$ 747	$ 765	$ 563	$ 963	$ 1,135	$ 1,215	$ 1,390	$ 1,171	$ 3,038	$ 4,911
Net income (loss)	$ (1,754)	$ (813)	$ (554)	$ (527)	$ (167)	$ (182)	$ 9	$ (96)	$ (3,648)	$ (436)
Income (loss) per common share	$ (1.92)	$ (0.96)	$ (0.60)	$ (0.60)	$ (0.12)	$ (0.24)	$ 0.01	$ (0.12)	$ (4.10)	$ (0.49)