Change in Accounting Principle (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2018 USD ($) item	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 31, 2016 USD ($)
Change in Accounting Principle
Warrant liability				$ 1,500,000		$ 1,900,000
Warrant derivative loss (gain)				400,000		(17,100,000)
Accumulated Deficit	$ (511,604,000)	$ (509,082,000)	$ (511,604,000)		$ (509,082,000)
Number of open contracts | item			1
Gain on sale of assets			$ 1,335,000	596,000
Trading securities		238,000			238,000
Available for sale equity unrealized losses					40,000
Retained earnings and other comprehensive income adjustment			(40,000)
Trading securities gain	61,000		67,000
ASU 2017-11 Earnings Per Share
Change in Accounting Principle
Warrant derivative loss (gain)			8,000	$ 1,000		$ 300,000
Sale, not discontinued operations | Zacatecas
Change in Accounting Principle
Total consideration	$ 1,500,000		1,500,000				$ 1,500,000
Gain on sale of assets			336,000
Sale, not discontinued operations | Zacatecas | ASU 2014-09 Revenue from Contracts with Customers
Change in Accounting Principle
Gain on sale of assets			$ 49,000
Sale, not discontinued operations | Zacatecas | Previously Reported
Change in Accounting Principle
Gain on sale of assets		49,000
Sale, not discontinued operations | Zacatecas | Cumulative Effect Adjustment | ASU 2014-09 Revenue from Contracts with Customers
Change in Accounting Principle
Accumulated Deficit		$ (49,000)			$ (49,000)