Annual report pursuant to Section 13 and 15(d)

Assets and Liabilities Measured at Fair Value (Details)

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Assets and Liabilities Measured at Fair Value (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Realized and Unrealized Gains (Losses) on Financial Instruments      
Realized and unrealized gains (losses) $ 3,098,000 $ 94,122,000 $ 2,619,000
Zero-strike call option      
Realized and Unrealized Gains (Losses) on Financial Instruments      
Realized and unrealized gains (losses) $ 3,098,000 1,132,000  
Time Warner Cable | Investment and financial instruments [member]      
Realized and Unrealized Gains (Losses) on Financial Instruments      
Realized and unrealized gains (losses)   $ 92,990,000 $ 2,619,000