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Summary of Significant Accounting Policies - Capitalized Software for Internal Use (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]      
Net capitalized software balance $ 3.5 $ 3.2  
Amortization expense related to capitalized software cost $ 1.3 $ 1.9 $ 1.7
Computer software | Minimum      
Finite-Lived Intangible Assets [Line Items]      
Useful life 3 years    
Computer software | Maximum      
Finite-Lived Intangible Assets [Line Items]      
Useful life 10 years