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, 2020 GBP (£)	Mar. 31, 2020 CAD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 CAD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2020 GBP (£)	Mar. 31, 2020 CAD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 CAD ($)
Denominated in Canadian Dollars | Designated as Hedging Instrument | Net Investment Hedging
Derivatives, Fair Value [Line Items]
Derivative asset							$ 700,000			$ 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 Pounds 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 Pounds Sterling | Derivative Instruments Not Designated | Purchase Contracts
Derivatives, Fair Value [Line Items]
Derivative asset | £						125,000			125,000
Denominated in Pounds Sterling | Derivative Instruments Not Designated | Sales Contracts
Derivatives, Fair Value [Line Items]
Derivative liability | £						£ (125,000)			£ (125,000)
Denominated in U.S Dollars | Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Derivative asset					$ 1,188,250			$ 1,188,250
Denominated in U.S Dollars | Derivative Instruments Not Designated
Derivatives, Fair Value [Line Items]
Derivative asset					$ 405,819			$ 405,819