Derivative Instrument (Notional Table) (Details) £ in Thousands, $ in Thousands, $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018 GBP (£)	Jun. 30, 2018 CAD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 CAD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 GBP (£)	Jun. 30, 2018 CAD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 CAD ($)
Canada, Dollars | Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Derivative, notional amount							$ 0			$ 36,000
Canada, Dollars | Designated as Hedging Instrument | Net Investment Hedging
Derivatives, Fair Value [Line Items]
Derivative, notional amount							575,000	$ 575,000
Notional amount of non-derivative instruments		$ 250,000		$ 250,000
United States of America, Dollars | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount					$ 408,007			408,007
United Kingdom, Pounds | Designated as Hedging Instrument | Net Investment Hedging
Derivatives, Fair Value [Line Items]
Derivative, notional amount | £						£ 891,640			£ 550,000
Notional amount of non-derivative instruments | £	£ 1,050,000		£ 1,050,000
Short | Canada, Dollars | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount							500,000			0
Short | United Kingdom, Pounds | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount					350,000			0
Long | Canada, Dollars | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount							$ 580,000			$ 80,000
Long | United Kingdom, Pounds | Not Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Derivative, notional amount					$ 350,000			$ 0