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Debt - Term Credit Agreement (Narrative) (Details) - USD ($)
Aug. 05, 2015
Dec. 31, 2017
New credit facilities    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 550,000,000  
Term Credit Agreement    
Debt Instrument [Line Items]    
Effective interest rate   3.00%
Term Credit Agreement | Term loan facility    
Debt Instrument [Line Items]    
Maximum borrowing capacity $ 350,000,000  
Debt instrument, term 9 years  
Basis spread on variable rate 1.625%  
Effective interest rate   3.30%