Derivative Instruments - Notional Amount of Derivatives and Other Financial Instruments (Details) £ in Thousands, $ in Thousands, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2020 CAD ($)	Sep. 30, 2020 GBP (£)	Dec. 31, 2019 CAD ($)	Dec. 31, 2019 GBP (£)	Sep. 30, 2020 GBP (£)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 USD ($)
Denominated in Canadian Dollars | Designated as Hedging Instrument | Net Investment Hedging
Derivatives, Fair Value [Line Items]
Derivative asset	$ 650,000		$ 725,000
Net investment hedges	250,000		250,000
Denominated in Canadian Dollars | Derivative Instruments Not Designated | Purchase Contracts
Derivatives, Fair Value [Line Items]
Derivative asset	$ 80,000		$ 0
Denominated in Pound Sterling | Designated as Hedging Instrument | Net Investment Hedging
Derivatives, Fair Value [Line Items]
Derivative asset | £					£ 1,340,708		£ 1,340,708
Net investment hedges | £		£ 1,050,000		£ 1,050,000
Denominated in Pound Sterling | Derivative Instruments Not Designated | Purchase Contracts
Derivatives, Fair Value [Line Items]
Derivative asset | £					0		125,000
Denominated in Pound Sterling | Derivative Instruments Not Designated | Sales Contracts
Derivatives, Fair Value [Line Items]
Derivative liability | £					£ 0		£ (125,000)
Denominated in U.S Dollars | Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Derivative asset						$ 450,000		$ 1,188,250
Denominated in U.S Dollars | Derivative Instruments Not Designated
Derivatives, Fair Value [Line Items]
Derivative asset						$ 95,086		$ 405,819