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Fair Value Measurements - Financial Liabilities not Carried at Fair Value (Details) - Level 2 - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Carrying Amount |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Senior notes | $ 745.7 | $ 945.1
Convertible securities | 309.9 | 307.5
Fair Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Senior notes | 765.2 | 936.0
Convertible securities | $ 549.8 | $ 466.9

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- Definition Fair value portion of notes payable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20,22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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