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Cash and Cash Equivalents and Short-term Investments (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2018
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Investments:          
Trading securities gain $ 61 $ 67      
Retained earnings and other comprehensive income adjustment   (40)      
Trading securities, cost 275 275     $ 275
Total short term, cost 275 275     275
Trading securities         238
Total short term 343 343     238
Financial institutions minimum net worth $ 1,000,000 $ 1,000,000      
Golden Tag          
Investments:          
Investment shares held (in shares) 7,500,000 7,500,000      
Ownership in property (as a percent)   10.00%      
Trading securities          
Investments:          
Trading securities, cost $ 275 $ 275      
Equity Securities          
Investments:          
Trading securities, cost         275
Estimated Fair Value.          
Investments:          
Trading securities 343 343     238
Total short term 343 343     238
Estimated Fair Value. | Trading securities          
Investments:          
Trading securities 343 343      
Estimated Fair Value. | Equity Securities          
Investments:          
Trading securities         238
Carrying Value.          
Investments:          
Trading securities 343 343     238
Total short term 343 343     238
Carrying Value. | Trading securities          
Investments:          
Trading securities $ 343 $ 343      
Carrying Value. | Equity Securities          
Investments:          
Trading securities         $ 238
San Diego | Golden Tag          
Investments:          
Ownership in property (as a percent)     50.00% 50.00%