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Marketable Investment Securities (Details 3) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Gain (Loss) on Securities [Line Items]          
Proceeds from Sale of Available-for-sale Securities $ 50,000,000 $ 0 $ 50,000,000.0 $ 8,900,000  
Available-for-sale Securities, Gross Realized Gains 0 $ 0 0 0  
Equity securities          
Gain (Loss) on Securities [Line Items]          
Available-for-sale Securities, Equity Securities, Current         $ 1,100,000
Available-for-sale Equity Securities, Amortized Cost Basis         1,500,000
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax         $ 400,000
Other Nonoperating Income (Expense) [Member] | Equity securities          
Gain (Loss) on Securities [Line Items]          
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net       $ 3,300,000  
Gain (Loss) on Investments [Member] | Equity securities          
Gain (Loss) on Securities [Line Items]          
Marketable Securities, Unrealized Gain (Loss), Excluding Other-than-temporary Impairment Loss $ 100,000   $ 300,000