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Derivative Financial Instruments - Additional Information (Details) - Forward contracts
£ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2018
GBP (£)
Aug. 28, 2018
$ / unit
Derivative [Line Items]        
Cash collateral held from counterparty | $ $ 3,100,000 $ 3,100,000    
Designated as Hedging Instrument | Net Investment Hedging        
Derivative [Line Items]        
Derivative notional amount 400,000,000 400,000,000 £ 285.8  
Hedge ineffectiveness recorded in investment and other income | $ $ 0 $ 0    
Designated as Hedging Instrument | Net Investment Hedging | Minimum        
Derivative [Line Items]        
Derivative, option strike price (in dollars per unit) | $ / unit       1.288
Designated as Hedging Instrument | Net Investment Hedging | Maximum        
Derivative [Line Items]        
Derivative, option strike price (in dollars per unit) | $ / unit       1.535