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Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Treasury money market funds, at fair value	$ 268.6	$ 131.4
Cost Method Investments, Fair Value Disclosure	$ 2.0

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- Definition

Investments in treasury money market funds which are part of the company's assets, measured at fair value.

+ References

No definition available.

+ Details

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- Definition

Fair value portion of investments accounted under the cost method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 325
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75030709&loc=d3e39828-111592

+ Details

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Period Type:	instant

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- References

No definition available.

+ Details

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