Derivative Instruments - Notional Amount of Derivatives and Other Financial Instruments (Details) £ in Thousands, $ in Thousands, $ in Thousands	Sep. 30, 2019 USD ($)	Sep. 30, 2019 CAD ($)	Sep. 30, 2019 GBP (£)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 CAD ($)	Dec. 31, 2018 GBP (£)
Denominated in Canadian Dollars | Designated as Hedging Instrument | Net Investment Hedging
Derivatives, Fair Value [Line Items]
Derivative asset		$ 725,000			$ 575,000
Derivative liability		250,000			250,000
Denominated in Canadian Dollars | Derivative Instruments Not Designated | Purchase Contracts
Derivatives, Fair Value [Line Items]
Derivative liability		200,000			325,000
Denominated in Canadian Dollars | Derivative Instruments Not Designated | Sales Contracts
Derivatives, Fair Value [Line Items]
Derivative asset		$ 237,000			$ 405,000
Denominated in U.S Dollars | Designated as Hedging Instrument | Cash Flow Hedging
Derivatives, Fair Value [Line Items]
Derivative liability	$ 1,188,250			$ 0
Denominated in U.S Dollars | Derivative Instruments Not Designated
Derivatives, Fair Value [Line Items]
Derivative asset	$ 405,819			$ 405,819
Denominated in Pounds Sterling | Designated as Hedging Instrument | Net Investment Hedging
Derivatives, Fair Value [Line Items]
Derivative asset | £			£ 1,340,708			£ 890,708
Derivative liability | £			1,050,000			1,050,000
Denominated in Pounds Sterling | Derivative Instruments Not Designated | Purchase Contracts
Derivatives, Fair Value [Line Items]
Derivative liability | £			125,000			350,000
Denominated in Pounds Sterling | Derivative Instruments Not Designated | Sales Contracts
Derivatives, Fair Value [Line Items]
Derivative asset | £			£ 125,000			£ 350,000