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Long-term Debt - Narrative - Senior Credit Facility (Details) - USD ($)
Jun. 30, 2018
Dec. 31, 2017
Jun. 17, 2016
Debt Instrument [Line Items]      
Outstanding balance $ 661,800,000 $ 670,900,000  
Line of Credit      
Debt Instrument [Line Items]      
Outstanding balance 358,600,000 $ 368,300,000  
Line of Credit | Senior Credit Facility      
Debt Instrument [Line Items]      
Maximum borrowing capacity     $ 750,000,000.0
Line of Credit | Revolving Credit Facility      
Debt Instrument [Line Items]      
Maximum borrowing capacity     350,000,000.0
Long-term line of credit 0    
Line of Credit | Term loan      
Debt Instrument [Line Items]      
Maximum borrowing capacity     400,000,000.0
Outstanding balance 360,000,000    
Line of Credit | Letter of Credit      
Debt Instrument [Line Items]      
Maximum borrowing capacity     100,000,000.0
Outstanding letters of credit $ 38,400,000    
Line of Credit | Bridge Loan      
Debt Instrument [Line Items]      
Maximum borrowing capacity     $ 10,000,000.0