Credit Facilities (Outstanding Letters Of Credit And Borrowings Of Bermuda Re Letter Of Credit Facility) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Line of Credit Facility [Line Items]
Commitment	$ 1,300,000,000	$ 1,300,000,000
In Use	127,000.000
Date of Expiry	9/30/2012
Citibank Bilateral Letter Of Credit Agreement One [Member]
Line of Credit Facility [Line Items]
Commitment	300,000,000	300,000,000
In Use	3,352,000	2,291,000
Date of Expiry	11/24/2012	11/24/2011
Citibank Bilateral Letter Of Credit Agreement Two [Member]
Line of Credit Facility [Line Items]
In Use	80,770,000	79,681,000
Date of Expiry	12/31/2012	12/31/2011
Citibank Bilateral Letter Of Credit Agreement Three [Member]
Line of Credit Facility [Line Items]
In Use	85,000	36,462,000
Date of Expiry	7/15/2013	1/31/2011
Citibank Bilateral Letter Of Credit Agreement Four [Member]
Line of Credit Facility [Line Items]
In Use	889,000	340,000
Date of Expiry	2/15/2014	6/15/2011
Citibank Bilateral Letter Of Credit Agreement Five [Member]
Line of Credit Facility [Line Items]
In Use	4,773,000	25,581,000
Date of Expiry	12/31/2014	6/30/2014
Citibank Bilateral Letter Of Credit Agreement Six [Member]
Line of Credit Facility [Line Items]
In Use	25,510,000	11,580,000
Date of Expiry	6/30/2015	9/30/2014
Citibank Bilateral Letter Of Credit Agreement Seven [Member]
Line of Credit Facility [Line Items]
In Use	8,642,000	72,398,000
Date of Expiry	9/30/2015	12/31/2014
Citibank Bilateral Letter Of Credit Agreement Eight [Member]
Line of Credit Facility [Line Items]
In Use	10,088,000
Date of Expiry	11/22/2015
Citibank Bilateral Letter Of Credit Agreement Nine [Member]
Line of Credit Facility [Line Items]
In Use	60,752,000
Date of Expiry	12/31/2015
Total Citibank Bilateral Agreement [Member]
Line of Credit Facility [Line Items]
Commitment	300,000,000	300,000,000
In Use	$ 194,861,000	$ 228,333,000