Derivative Instruments - Notional Amount of Derivatives and Other Financial Instruments (Details) £ in Thousands, $ in Thousands, $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2021 CAD ($)	Jun. 30, 2021 GBP (£)	Dec. 31, 2020 CAD ($)	Dec. 31, 2020 GBP (£)	Jun. 30, 2021 GBP (£)	Jun. 30, 2021 USD ($)	Dec. 31, 2020 GBP (£)	Dec. 31, 2020 USD ($)
Denominated in Canadian Dollars | Designated as Hedging Instrument | Net Investment Hedging
Derivatives, Fair Value [Line Items]
Derivative asset	$ 625,000		$ 625,000
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		$ 80,000
Denominated in Pound Sterling | Designated as Hedging Instrument | Net Investment Hedging
Derivatives, Fair Value [Line Items]
Derivative asset | £					£ 1,880,708		£ 1,340,708
Net investment hedges | £		£ 1,050,000		£ 1,050,000
Denominated in U.S Dollars | Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Derivative asset						$ 25,000		$ 450,000
Denominated in U.S Dollars | Derivative Instruments Not Designated
Derivatives, Fair Value [Line Items]
Derivative asset						$ 26,137		$ 26,137