Credit Facilities (Summary Of Outstanding Letters Of Credit And/Or Borrowings) (Details) £ in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016 GBP (£)	Dec. 31, 2015 GBP (£)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Line of Credit Facility [Line Items]
Commitment			$ 1,100,000
Wells Fargo Bank Group Credit Facility Tranche One [Member]
Line of Credit Facility [Line Items]
Commitment			200,000	$ 200,000
Wells Fargo Bank Group Credit Facility Tranche Two Issuance One [Member]
Line of Credit Facility [Line Items]
Commitment			600,000	600,000
In Use			2,488	2,488
Date of Expiry	Dec. 12, 2016	Dec. 12, 2016
Wells Fargo Bank Group Credit Facility Tranche Two Issuance Two [Member]
Line of Credit Facility [Line Items]
In Use			452,552	447,178
Date of Expiry	Dec. 31, 2016	Dec. 31, 2016
Wells Fargo Bank Group Credit Facility [Member]
Line of Credit Facility [Line Items]
Commitment			800,000	800,000
In Use			455,040	449,666
Citibank Bilateral Letter Of Credit Agreement One [Member]
Line of Credit Facility [Line Items]
Commitment			300,000	300,000
In Use			3,672	3,672
Date of Expiry	Nov. 24, 2016	Nov. 24, 2016
Citibank Bilateral Letter Of Credit Agreement Two [Member]
Line of Credit Facility [Line Items]
In Use			64,823	67,783
Date of Expiry	Dec. 31, 2016	Dec. 31, 2016
Citibank Bilateral Letter Of Credit Agreement Three [Member]
Line of Credit Facility [Line Items]
In Use			4,058	179
Date of Expiry	Feb. 28, 2017	Aug. 30, 2017
Citibank Bilateral Letter Of Credit Agreement Four [Member]
Line of Credit Facility [Line Items]
In Use			184	316
Date of Expiry	Aug. 30, 2017	Dec. 31, 2017
Citibank Bilateral Letter Of Credit Agreement Five [Member]
Line of Credit Facility [Line Items]
In Use			2,598
Date of Expiry	Dec. 31, 2017
Citibank Bilateral Letter Of Credit Agreement Six [Member]
Line of Credit Facility [Line Items]
In Use			96,738	99,521
Date of Expiry	Mar. 30, 2020	Dec. 31, 2019
Citibank Bilateral Agreement [Member]
Line of Credit Facility [Line Items]
Commitment			300,000	300,000
In Use			$ 172,073	$ 171,471
Lloyd's Bank PLC [Member]
Line of Credit Facility [Line Items]
Commitment | £	£ 175,000	£ 175,000
In Use | £	£ 164,961	£ 164,961
Date of Expiry	Dec. 31, 2019	Dec. 31, 2019
Lloyd's of London Bank Credit Facility [Member]
Line of Credit Facility [Line Items]
Commitment | £	£ 175,000	£ 175,000
In Use | £	£ 164,961	£ 164,961