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Investments in Available-for-Sale Securities (Schedule of Investments) (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale, Cost Basis $ 164,630 $ 163,140
Available-for-sale, Gains 156 15
Available-for-sale, Losses (122) (358)
Available-for-sale, Fair Value 164,664 162,797
Available-for-sale, Short-Term Investments 164,664 162,797
Short-term Investments    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale, Short-Term Investments 164,664 162,797
Other Assets    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale, Other Assets 0 0
U.S. government and agency securities    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale, Cost Basis 94,586 100,003
Available-for-sale, Gains 58 11
Available-for-sale, Losses (37) (126)
Available-for-sale, Fair Value 94,607 99,888
U.S. government and agency securities | Short-term Investments    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale, Short-Term Investments 94,607 99,888
U.S. government and agency securities | Other Assets    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale, Other Assets 0 0
Corporate notes and bonds    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale, Cost Basis 70,044 63,137
Available-for-sale, Gains 98 4
Available-for-sale, Losses (85) (232)
Available-for-sale, Fair Value 70,057 62,909
Corporate notes and bonds | Short-term Investments    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale, Short-Term Investments 70,057 62,909
Corporate notes and bonds | Other Assets    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale, Other Assets $ 0 $ 0