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Investments (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2020
Jun. 30, 2019
Apr. 30, 2020
Dec. 31, 2019
Schedule of Investments [Line Items]          
Equity method investments $ 68,200 $ 68,200     $ 69,100
Investments in marketable equity securities 43,300 43,300     4,400
Unrealized gain on marketable securities 15,000 14,000      
Investments in non-marketable equity securities $ 16,800 16,800     $ 61,800
Write-down of non-marketable equity securities and note receivable   $ 20,000 $ 17,741    
Reclassified from Non-marketable Security to Marketable Security          
Schedule of Investments [Line Items]          
Investments in marketable equity securities       $ 25,000