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Fair Value Measurements (Schedule of Effective Maturity Dates of Held to Maturity Investments) (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Investment maturity    
Held-to-maturity maturities, less than 1 year, book value $ 548,196 $ 514,835
Held-to-maturity maturities, less than 1 year, fair value $ 548,141 $ 514,829