Marketable Securities (Available-for-sale Debt Securities by Contractual Maturity) (Details) $ in Millions	Dec. 31, 2019 USD ($)
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	$ 365.5
Available-for-sale Securities, Debt Maturities, Year Two Through Three, Fair Value	3.5
Available-for-sale Securities, Debt Maturities, Year Four Through Five, Fair Value	0.0
Debt Securities, Available-for-sale	$ 369.0

Available-for-sale Securities, Debt Maturities, Year Four Through Five, Fair Value

No definition available.

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- Definition

Available-for-sale Securities, Debt Maturities, Year Two Through Three, Fair Value

+ References

No definition available.

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- Definition

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing in next fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

+ References

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-Publisher FASB
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-Publisher FASB
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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