Derivative Instruments (Notional Amount of Derivatives and Other Financial Instruments) (Details) £ in Thousands, $ in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019 CAD ($)	Mar. 31, 2019 GBP (£)	Dec. 31, 2020 CAD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2020 GBP (£)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 CAD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 USD ($)
Designated as Hedging Instrument | Net Investment Hedging | Denominated in Canadian Dollars
Derivatives, Fair Value [Line Items]
Derivative asset			$ 625,000				$ 725,000
Notional amount of non-derivative instruments	$ 250,000		250,000
Designated as Hedging Instrument | Net Investment Hedging | Denominated in Pound Sterling
Derivatives, Fair Value [Line Items]
Derivative asset | £					£ 1,340,708			£ 1,340,708
Notional amount of non-derivative instruments | £		£ 1,050,000		£ 1,050,000
Designated as Hedging Instrument | Cash Flow Hedging | Denominated in U.S. Dollars
Derivatives, Fair Value [Line Items]
Derivative asset						$ 450,000			$ 1,188,250
Not Designated as Hedging Instrument | Denominated in Canadian Dollars | Short
Derivatives, Fair Value [Line Items]
Derivative asset			$ 80,000				$ 0
Not Designated as Hedging Instrument | Denominated in U.S. Dollars
Derivatives, Fair Value [Line Items]
Derivative asset						$ 26,137			$ 405,819
Not Designated as Hedging Instrument | Denominated in Pound Sterling | Long
Derivatives, Fair Value [Line Items]
Derivative liability | £					0			(125,000)
Not Designated as Hedging Instrument | Denominated in Pound Sterling | Short
Derivatives, Fair Value [Line Items]
Derivative asset | £					£ 0			£ 125,000