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Debt and Credit Facilities (Schedule of Credit Facilities) (Details) - Dec. 31, 2017
£ in Thousands, $ in Thousands
USD ($)
GBP (£)
Line of Credit Facility [Line Items]    
Credit facilities outstanding $ 487,857 £ 10,000
Wells Fargo | Standby Letter of Credit Facility | Letter of Credit    
Line of Credit Facility [Line Items]    
Credit facilities outstanding 106,794  
National Australia Bank Limited | Standby Letter of Credit Facility | Letter of Credit    
Line of Credit Facility [Line Items]    
Credit facilities outstanding 3,785  
Revolving Credit Facility    
Line of Credit Facility [Line Items]    
Credit facilities outstanding $ 0