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Debt Obligations - Bridge Credit Agreement (Details)
Oct. 20, 2017
USD ($)
Sep. 30, 2017
GBP (£)
Jul. 25, 2017
GBP (£)
Term Loan Facility      
Debt Instrument [Line Items]      
Bridge loan $ 0 £ 344,200,000 £ 1,115,000,000.000
Senior Notes      
Debt Instrument [Line Items]      
Stated interest rate percentage 4.00%    
Senior Notes | 4.00% Senior Notes      
Debt Instrument [Line Items]      
Aggregate principal amount $ 450,000,000.0    
Stated interest rate percentage 4.00%