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Long-Term Debt (Details 3)
£ in Millions
12 Months Ended
Oct. 13, 2017
Dec. 31, 2017
USD ($)
Rate
Dec. 31, 2017
GBP (£)
Dec. 31, 2016
USD ($)
Debt Instrument [Line Items]        
Credit Facility, Covenant Terms, Maximum Debt to Capitalization Ratio   The company also cannot permit the ratio of its debt to capitalization (as set forth in the credit agreements) to exceed 65 percent.    
Line of Credit Facility, Covenant Compliance   the company was in compliance with all covenants under its credit agreements.    
Five Year Term [Member] [Domain]        
Debt Instrument [Line Items]        
Line of Credit Facility, Maximum Borrowing Capacity   $ 1,600,000,000    
Credit facilities   0    
Two Year Term [Member] [Domain]        
Debt Instrument [Line Items]        
Credit facilities   134,000,000   $ 135,000,000
Two Year Term [Member] [Domain] | Revolving Credit Facility [Member]        
Debt Instrument [Line Items]        
Line of Credit Facility, Maximum Borrowing Capacity   $ 161,000,000 £ 120  
Credit facilities | £     £ 100  
London Interbank Offered Rate (LIBOR) [Member] | Two Year Term [Member] [Domain]        
Debt Instrument [Line Items]        
Line of Credit Facility, Interest Rate During Period | Rate   1.10%    
GeneralRedemption [Domain]        
Debt Instrument [Line Items]        
Debt Instrument, Redemption Price, Percentage 100.00%      
MandatoryRedemption [Domain]        
Debt Instrument [Line Items]        
Debt Instrument, Redemption Price, Percentage 101.00%