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Significant Accounting Policies and Consolidated Financial Statement Components Property at cost, net of depreciation (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items]    
Other Assets, Miscellaneous $ 30.4  
Tangible Asset Impairment Charges 0.0 $ 0.6
Cost-method Investments, Other than Temporary Impairment $ 2.1  
Furniture and Fixtures [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 5 years  
Computer Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, Useful Life 3 years