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Investments in Debt and Equity Securities (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Investment Holdings [Line Items]    
Held to maturity $ 42,316 $ 305,781
Available for sale 38,895 31,197
Total 81,211 336,978
Held to maturity, short-term investments 42,316 305,781
Available for sale, short-term investments 1,200 1,200
Total, short-term investments 43,516 306,981
Held to maturity, long-term investments 0 0
Available for sale, long-term investments 37,695 29,997
Total, long-term investments 37,695 29,997
Certificates of deposit, commercial paper and money market funds due within one year    
Investment Holdings [Line Items]    
Held to maturity 42,316 255,781
Available for sale, debt securities 0 0
Total 42,316 255,781
Investments in mutual funds and common stock    
Investment Holdings [Line Items]    
Held to maturity 0 50,000
Available for sale, equity securities 38,895 31,197
Total $ 38,895 $ 81,197