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Derivative Instruments - Summary of Non-Designated Foreign Exchange Forwards (Details) - Not Designated as Hedging Instrument [Member] - Foreign Currency Contract [Member]
Sep. 30, 2017
GBP (£)
contract
Dec. 31, 2016
GBP (£)
contract
Derivative [Line Items]    
Number of Contracts | contract 13 11
Aggregate Notional Amount | £ £ 197,401,000 £ 148,310,000