Derivative Instruments - Notional Amounts (Details) - Derivatives designated as hedging instruments - Cash flow hedges € in Thousands, £ in Thousands, $ in Thousands	Mar. 31, 2018 GBP (£)	Mar. 31, 2018 EUR (€)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 USD ($)
Foreign currency exchange forward contracts
Notional amounts of outstanding derivatives instruments designated as cash flow hedges
Notional amount	£ 60,665	€ 20,476		£ 46,465	€ 5,350
Foreign currency exchange purchased option contracts
Notional amounts of outstanding derivatives instruments designated as cash flow hedges
Notional amount	£ 37,140	€ 2,650		£ 34,050
Interest rate swap contracts
Notional amounts of outstanding derivatives instruments designated as cash flow hedges
Notional amount			$ 43,935			$ 44,756