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Derivative Instruments - Summary of Non-Designated Foreign Exchange Forwards (Details) - Not Designated as Hedging Instrument [Member] - Forward Currency Contract [Member]
Dec. 31, 2016
GBP (£)
contract
Dec. 31, 2015
GBP (£)
contract
Derivative [Line Items]    
Derivative, Number of Instruments Held | contract 11 5
Derivative, notional amount | £ £ 148,310 £ 130,272