Derivative Instruments - Notional Amounts (Details) - Derivatives designated as hedging instruments - Cash flow hedges £ in Thousands, $ in Thousands	Sep. 30, 2017 GBP (£)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 GBP (£)
Foreign currency exchange forward contracts
Notional amounts of outstanding derivatives instruments designated as cash flow hedges
Notional amount	£ 42,570		£ 25,270
Foreign currency exchange purchased option contracts
Notional amounts of outstanding derivatives instruments designated as cash flow hedges
Notional amount	£ 28,960		£ 8,160
Interest rate swap contracts
Notional amounts of outstanding derivatives instruments designated as cash flow hedges
Notional amount | $		$ 45,578