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Organization and Significant Accounting Policies - Property and Equipment, net (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]      
Depreciation $ 3.4 $ 3.7 $ 4.4
Capital lease amortization expense $ 1.6 $ 2.1 $ 2.4
Minimum      
Property, Plant and Equipment [Line Items]      
Property and equipment, useful life 3 years    
Maximum      
Property, Plant and Equipment [Line Items]      
Property and equipment, useful life 10 years