Marketable Securities - Narrative (Details) $ in Millions	Mar. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Investments, Debt and Equity Securities [Abstract]
Debt securities held in an unrealized loss position for less than 12 months \| security	17	24
Fair value of debt securities in an unrealized loss position for less than 12 months	$ 132.7	$ 177.1
Aggregate fair value of marketable securities maturing within one year	132.7
Aggregate fair value of marketable securities maturing after one year through two years	$ 0.0

Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth 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.

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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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 3
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months.

+ References

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-Publisher FASB
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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

+ References

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-Publisher FASB
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