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Fair Value (Narrative) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($)
Fair Value Disclosure [Line Items] |
Transfers between Fair Value Hierarchies | 0
Fair Value, Assets and Liabilities Measured on a Non-Recurring Basis | 0
U.S. Treasury debt securities |
Fair Value Disclosure [Line Items] |
Available-for-sale securities, maturities of less than a year | $ 2.9
Corporate/Other debt securities |
Fair Value Disclosure [Line Items] |
Available-for-sale securities, maturities of less than a year | $ 2.7

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