Credit Facilities (Summary Of Outstanding Letters Of Credit And Borrowings) (Details)	12 Months Ended
	Dec. 31, 2020 GBP (£)	Dec. 31, 2019 GBP (£)	Dec. 31, 2020 USD ($)	Mar. 25, 2020 USD ($)	Dec. 31, 2019 USD ($)	Nov. 07, 2019 GBP (£)	May 26, 2016 USD ($)	Jun. 22, 2012 USD ($)
Line of Credit Facility [Line Items]
Commitment	£ 52,175,000		$ 1,000,000,000
Wells Fargo Bank Group Credit Facility [Member]
Line of Credit Facility [Line Items]
Commitment			800,000,000		$ 800,000,000		$ 800,000,000	$ 800,000,000
In Use			753,932,000		623,484,000
Wells Fargo Bank Group Credit Facility Tranche One [Member]
Line of Credit Facility [Line Items]
Commitment			200,000,000
In Use				$ 50,000,000	0
Wells Fargo Bank Group Credit Facility Tranche One [Member] \| Debt Instrument Redemption Period One [Member]
Line of Credit Facility [Line Items]
Commitment			200,000,000		200,000,000
In Use			164,242,000		33,737,000
Date of Expiry	Dec. 31, 2021	Dec. 31, 2020
Wells Fargo Bank Group Credit Facility Tranche Two [Member]
Line of Credit Facility [Line Items]
Commitment			600,000,000
Wells Fargo Bank Group Credit Facility Tranche Two [Member] \| Debt Instrument Redemption Period One [Member]
Line of Credit Facility [Line Items]
Commitment			600,000,000		600,000,000
In Use			589,690,000		2,381,000
Date of Expiry	Dec. 31, 2021	Jul. 29, 2020
Wells Fargo Bank Group Credit Facility Tranche Two [Member] \| Debt Instrument Redemption Period Two [Member]
Line of Credit Facility [Line Items]
In Use					1,649,000
Date of Expiry		Sep. 30, 2020
Wells Fargo Bank Group Credit Facility Tranche Two [Member] \| Debt Instrument Redemption Period Three [Member]
Line of Credit Facility [Line Items]
In Use					573,353,000
Date of Expiry		Dec. 31, 2020
Wells Fargo Bank Group Credit Facility Tranche Two [Member] \| Debt Instrument Redemption Period Four [Member]
Line of Credit Facility [Line Items]
In Use					12,364,000
Date of Expiry		Jan. 04, 2021
Citibank Bilateral Agreement [Member]
Line of Credit Facility [Line Items]
Commitment			200,000,000		200,000,000
In Use			185,483,000		147,970,000
Citibank Bilateral Letter Of Credit Agreement One [Member]
Line of Credit Facility [Line Items]
Commitment			200,000,000		200,000,000
In Use			4,425,000		4,425,000
Date of Expiry	Feb. 28, 2021	Feb. 29, 2020
Citibank Bilateral Letter Of Credit Agreement Two [Member]
Line of Credit Facility [Line Items]
In Use			3,672,000		512,000
Date of Expiry	Nov. 24, 2021	Sep. 03, 2020
Citibank Bilateral Letter Of Credit Agreement Three [Member]
Line of Credit Facility [Line Items]
In Use			448,000		3,672,000
Date of Expiry	Dec. 16, 2021	Nov. 24, 2020
Citibank Bilateral Letter Of Credit Agreement Four [Member]
Line of Credit Facility [Line Items]
In Use			115,000		177,000
Date of Expiry	Dec. 20, 2021	Dec. 16, 2020
Citibank Bilateral Letter Of Credit Agreement Five [Member]
Line of Credit Facility [Line Items]
In Use			136,383,000		125,000
Date of Expiry	Dec. 31, 2021	Dec. 20, 2020
Citibank Bilateral Letter Of Credit Agreement Six [Member]
Line of Credit Facility [Line Items]
In Use			39,619,000		101,404,000
Date of Expiry	Dec. 30, 2024	Dec. 31, 2020
Citibank Bilateral Letter Of Credit Agreement Seven [Member]
Line of Credit Facility [Line Items]
In Use			821,000		559,000
Date of Expiry	Aug. 15, 2022	Aug. 15, 2021
Citibank Bilateral Letter Of Credit Agreement Eight [Member]
Line of Credit Facility [Line Items]
In Use					$ 37,096,000
Date of Expiry		Dec. 30, 2023
Lloyds Bank plc Credit Facility [Member]
Line of Credit Facility [Line Items]
Commitment \| £	£ 52,175,000	£ 47,000,000				£ 52,175,000
In Use \| £	52,175,000	47,000,000
Lloyd's Bank Issuance One [Member]
Line of Credit Facility [Line Items]
Commitment \| £
In Use \| £
Lloyd S Bank Plc [Member]
Line of Credit Facility [Line Items]
Commitment \| £	52,175,000	47,000,000
In Use \| £	£ 52,175,000	£ 47,000,000
Date of Expiry	Dec. 31, 2024	Dec. 31, 2023