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Short-term Investments Marketable Securities (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Proceeds from Sale of Debt Securities, Available-for-sale $ 65,144,000 $ 467,790,000
Debt Securities, Trading, Amortized Cost 33,643,000  
Debt Securities, Trading, Unrealized Gain 18,000  
Debt Securities, Trading, Unrealized Loss   19,000
Debt Securities, Trading 33,642,000 0
Available-for-sale Securities, Amortized Cost Basis 37,032,000  
Available-for-sale Securities, Gross Unrealized Gain   45,000
Available-for-sale Securities, Gross Unrealized Loss   423,000
Available-for-sale Securities 36,654,000  
Trading Securities, Equity, Cost 3,389,000  
Trading Securities, Unrealized Holding Gain 27,000  
Trading Securities, Unrealized Holding Loss (404,000)  
Trading Securities, Equity 3,012,000 $ 0
Proceeds from Sale of Available-for-sale Securities, Equity 1,496,000  
Fair Value, Inputs, Level 1 [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Trading 0  
Trading Securities, Equity $ 3,012,000