Derivative Instruments (Notional Amount of Derivatives and Other Financial Instruments) (Details) £ in Thousands, $ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 CAD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 CAD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 CAD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 GBP (£)	Dec. 31, 2018 CAD ($)
Designated as Hedging Instrument | Net Investment Hedging | Denominated in Canadian Dollars
Derivatives, Fair Value [Line Items]
Derivative asset							$ 725,000			$ 575,000
Notional amount of non-derivative instruments		$ 250,000		$ 250,000
Designated as Hedging Instrument | Net Investment Hedging | Denominated in Pounds Sterling
Derivatives, Fair Value [Line Items]
Derivative asset | £						£ 1,340,708			£ 890,708
Notional amount of non-derivative instruments | £	£ 1,050,000		£ 1,050,000
Designated as Hedging Instrument | Cash Flow Hedging | Denominated in U.S. Dollars(1)
Derivatives, Fair Value [Line Items]
Derivative asset					$ 1,188,250			$ 0
Not Designated as Hedging Instrument | Denominated in Canadian Dollars | Long
Derivatives, Fair Value [Line Items]
Derivative liability							0			(325,000)
Not Designated as Hedging Instrument | Denominated in Canadian Dollars | Short
Derivatives, Fair Value [Line Items]
Derivative asset							$ 0			$ 405,000
Not Designated as Hedging Instrument | Denominated in U.S. Dollars(1)
Derivatives, Fair Value [Line Items]
Derivative asset					$ 405,819			$ 405,819
Not Designated as Hedging Instrument | Denominated in Pounds Sterling | Long
Derivatives, Fair Value [Line Items]
Derivative liability | £						(125,000)			(350,000)
Not Designated as Hedging Instrument | Denominated in Pounds Sterling | Short
Derivatives, Fair Value [Line Items]
Derivative asset | £						£ 125,000			£ 350,000