Credit Facilities (Summary Of Outstanding Letters Of Credit And/Or Borrowings) (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2020 GBP (£)	Dec. 31, 2019 GBP (£)	Mar. 31, 2020 USD ($)	Mar. 25, 2020 USD ($)	Dec. 31, 2019 USD ($)	May 26, 2016 USD ($)	Jun. 22, 2012 USD ($)
Line of Credit Facility [Line Items]
Commitment	£ 47,000,000		$ 1,000,000,000
In Use			50,000,000
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			50,000,000
Date of Expiry	May 26, 2020
Wells Fargo Bank Group Credit Facility Tranche One [Member] \| Debt Instrument Redemption Period Two [Member]
Line of Credit Facility [Line Items]
In Use			91,816,000		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			5,161,000		2,381,000
Date of Expiry	Jul. 29, 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		1,649,000
Date of Expiry	Sep. 30, 2020	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			569,136,000		573,353,000
Date of Expiry	Dec. 31, 2020	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			13,215,000		12,364,000
Date of Expiry	Jan. 04, 2021	Jan. 04, 2021
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			730,977,000		623,484,000
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			93,578,000		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			493,000		177,000
Date of Expiry	Aug. 15, 2021	Dec. 16, 2020
Citibank Bilateral Letter Of Credit Agreement Five [Member]
Line of Credit Facility [Line Items]
In Use			156,000		125,000
Date of Expiry	Dec. 16, 2021	Dec. 20, 2020
Citibank Bilateral Letter Of Credit Agreement Six [Member]
Line of Credit Facility [Line Items]
In Use			110,000		101,404,000
Date of Expiry	Dec. 20, 2021	Dec. 31, 2020
Citibank Bilateral Letter Of Credit Agreement Seven [Member]
Line of Credit Facility [Line Items]
In Use			4,663,000		559,000
Date of Expiry	Dec. 31, 2021	Aug. 15, 2021
Citibank Bilateral Letter Of Credit Agreement Eight [Member]
Line of Credit Facility [Line Items]
In Use			166,000		37,096,000
Date of Expiry	Jan. 21, 2024	Dec. 30, 2023
Citibank Bilateral Letter Of Credit Agreement Nine [Member]
Line of Credit Facility [Line Items]
In Use			36,413,000
Date of Expiry	Mar. 30, 2024
Citibank Bilateral Agreement [Member]
Line of Credit Facility [Line Items]
Commitment			200,000,000		200,000,000
In Use			$ 143,676,000		$ 147,970,000
Lloyds Bank PLC [Member]
Line of Credit Facility [Line Items]
Commitment \| £	£ 47,000,000	£ 47,000,000
In Use \| £	£ 47,000,000	£ 47,000,000
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 \| £	47,000,000	47,000,000
In Use \| £	£ 47,000,000	£ 47,000,000