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Investments in Available-for-Sale Securities (Cost Basis to Fair Value Reconciliation) (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Cost Basis $ 163,140 $ 156,189
Gross Unrealized Gains 15 22
Gross Unrealized Losses (358) (365)
Fair Value 162,797 155,846
U.S. government and agency securities    
Debt Securities, Available-for-sale [Line Items]    
Cost Basis 100,003 64,151
Gross Unrealized Gains 11 22
Gross Unrealized Losses (126) (73)
Fair Value 99,888 64,100
Corporate notes and bonds    
Debt Securities, Available-for-sale [Line Items]    
Cost Basis 63,137 92,038
Gross Unrealized Gains 4 0
Gross Unrealized Losses (232) (292)
Fair Value 62,909 91,746
Short-term investments    
Debt Securities, Available-for-sale [Line Items]    
Fair Value 162,797 155,846
Short-term investments | U.S. government and agency securities    
Debt Securities, Available-for-sale [Line Items]    
Fair Value 99,888 64,100
Short-term investments | Corporate notes and bonds    
Debt Securities, Available-for-sale [Line Items]    
Fair Value 62,909 91,746
Other assets    
Debt Securities, Available-for-sale [Line Items]    
Fair Value $ 0 0
Other assets | U.S. government and agency securities    
Debt Securities, Available-for-sale [Line Items]    
Fair Value   0
Other assets | Corporate notes and bonds    
Debt Securities, Available-for-sale [Line Items]    
Fair Value   $ 0