Derivative Instruments (Notional Amount of Derivatives and Other Financial Instruments) (Details) £ in Thousands, $ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 CAD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 CAD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 CAD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 CAD ($)
Designated as Hedging Instrument | Net Investment Hedging | Canada, Dollars
Derivatives, Fair Value [Line Items]
Derivative, notional amount							$ 575,000			$ 575,000
Notional amount of non-derivative instruments		$ 250,000		$ 250,000
Designated as Hedging Instrument | Net Investment Hedging | United Kingdom, Pounds
Derivatives, Fair Value [Line Items]
Derivative, notional amount | £						£ 890,708			£ 550,000
Notional amount of non-derivative instruments | £	£ 1,050,000		£ 1,050,000
Designated as Hedging Instrument | Cash Flow Hedging | Canada, Dollars
Derivatives, Fair Value [Line Items]
Derivative, notional amount							0			36,000
Not Designated as Hedging Instrument | United States of America, Dollars
Derivatives, Fair Value [Line Items]
Derivative, notional amount					$ 405,819			$ 408,007
Not Designated as Hedging Instrument | Canada, Dollars | Short [Member]
Derivatives, Fair Value [Line Items]
Derivative, notional amount							325,000			0
Not Designated as Hedging Instrument | Canada, Dollars | Long [Member]
Derivatives, Fair Value [Line Items]
Derivative, notional amount							$ 405,000			$ 80,000
Not Designated as Hedging Instrument | United Kingdom, Pounds | Short [Member]
Derivatives, Fair Value [Line Items]
Derivative, notional amount | £						350,000			0
Not Designated as Hedging Instrument | United Kingdom, Pounds | Long [Member]
Derivatives, Fair Value [Line Items]
Derivative, notional amount | £						£ 350,000			£ 0