Credit Facilities (Summary Of Outstanding Letters Of Credit And/Or Borrowings) (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2020 GBP (£)	Dec. 31, 2019 GBP (£)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 GBP (£)	Mar. 25, 2020 USD ($)	Dec. 31, 2019 USD ($)	May 26, 2016 USD ($)	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			685,263,000			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			99,077,000			33,737,000
Date of Expiry	Dec. 31, 2020	Dec. 31, 2020
Wells Fargo Bank Group Credit Facility Tranche Two [Member]
Line of Credit Facility [Line Items]
Commitment			600,000,000			600,000,000
Wells Fargo Bank Group Credit Facility Tranche Two [Member] | Debt Instrument Redemption Period One [Member]
Line of Credit Facility [Line Items]
In Use			586,186,000			2,381,000
Date of Expiry	Dec. 31, 2020	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 Letter Of Credit Agreement One [Member]
Line of Credit Facility [Line Items]
Commitment			200,000,000			200,000,000
In Use			3,672,000			4,425,000
Date of Expiry	Nov. 24, 2020	Feb. 29, 2020
Citibank Bilateral Letter Of Credit Agreement Two [Member]
Line of Credit Facility [Line Items]
In Use			93,846,000			512,000
Date of Expiry	Dec. 31, 2020	Sep. 03, 2020
Citibank Bilateral Letter Of Credit Agreement Three [Member]
Line of Credit Facility [Line Items]
In Use			4,425,000			3,672,000
Date of Expiry	Feb. 28, 2021	Nov. 24, 2020
Citibank Bilateral Letter Of Credit Agreement Four [Member]
Line of Credit Facility [Line Items]
In Use			183,000			177,000
Date of Expiry	Dec. 16, 2021	Dec. 16, 2020
Citibank Bilateral Letter Of Credit Agreement Five [Member]
Line of Credit Facility [Line Items]
In Use			109,000			125,000
Date of Expiry	Dec. 20, 2021	Dec. 20, 2020
Citibank Bilateral Letter Of Credit Agreement Six [Member]
Line of Credit Facility [Line Items]
In Use			5,475,000			101,404,000
Date of Expiry	Dec. 31, 2021	Dec. 31, 2020
Citibank Bilateral Letter Of Credit Agreement Seven [Member]
Line of Credit Facility [Line Items]
In Use			777,000			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			37,802,000			37,096,000
Date of Expiry	Sep. 30, 2024	Dec. 30, 2023
Citibank Bilateral Agreement [Member]
Line of Credit Facility [Line Items]
Commitment			200,000,000			200,000,000
In Use			$ 146,289,000			$ 147,970,000
Lloyds Bank PLC [Member]
Line of Credit Facility [Line Items]
Commitment | £	£ 52,175,000	£ 47,000,000
In Use | £		£ 47,000,000		£ 52,175,000
Date of Expiry	Dec. 31, 2023	Dec. 31, 2023
Lloyd's Bank Issuance One [Member]
Line of Credit Facility [Line Items]
Commitment | £
In Use | £
Lloyds Bank plc Credit Facility [Member]
Line of Credit Facility [Line Items]
Commitment | £	£ 52,175,000	47,000,000
In Use | £	£ 52,175,000	£ 47,000,000