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Marketable Investment Securities, Restricted Cash and Cash Equivalents, and Other Investment Securities - Changes in Estimated Fair Value (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair value of marketable investment securities in a loss position    
Total $ 647,378 $ 444,485
Unrealized loss on marketable investment securities in a loss position    
Total (920) (276)
Debt securities    
Fair value of marketable investment securities in a loss position    
Less than 12 Months 647,241 410,145
12 Months or More 137 34,340
Unrealized loss on marketable investment securities in a loss position    
Less than 12 Months (918) (156)
12 Months or More $ (2) $ (120)