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Fair Value Measurements - Summary of Financial Assets and Liabilities Not Carried at Fair Value (Details) (Level 2, USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Carrying Value
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes $ 1,084.2us-gaap_NotesPayableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
$ 736.8us-gaap_NotesPayableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementBasisAxis
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Convertible securities 303.6us-gaap_ConvertibleDebtFairValueDisclosures
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ us-gaap_FairValueByMeasurementBasisAxis
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303.1us-gaap_ConvertibleDebtFairValueDisclosures
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/ us-gaap_FairValueByMeasurementBasisAxis
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Fair Value
   
Fair Value, Balance Sheet Grouping, Financial Statement Captions    
Senior notes 1,136.1us-gaap_NotesPayableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementBasisAxis
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786.2us-gaap_NotesPayableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementBasisAxis
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Convertible securities $ 525.6us-gaap_ConvertibleDebtFairValueDisclosures
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_FairValueByMeasurementBasisAxis
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$ 532.1us-gaap_ConvertibleDebtFairValueDisclosures
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/ us-gaap_FairValueByMeasurementBasisAxis
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