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Summary of Significant Accounting Policies - Payroll Tax Credit (Details)
€ in Millions, $ in Millions
1 Months Ended 12 Months Ended
Mar. 31, 2016
USD ($)
Mar. 31, 2016
EUR (€)
Jul. 31, 2015
USD ($)
Jul. 31, 2015
EUR (€)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Accounting Policies [Abstract]                  
Multiplier for payroll tax credits           2.5      
Payment period           3 years      
Net proceeds from sale of tax credits $ 143.1 € 129.9 $ 132.8 € 120.1          
Tax Credit Carryforward [Line Items]                  
Percentage of eligible wages for tax credit           6.00% 6.00% 6.00% 4.00%
Scenario, Forecast                  
Tax Credit Carryforward [Line Items]                  
Percentage of eligible wages for tax credit         7.00%