Investments (Schedule Of Company's Trust Accounts On Deposit, Outstanding And Remaining Letter Of Credit Facilities, Collateral Committed To Support The Letter Of Credit Facilities) (Details) (USD $) In Thousands	Sep. 30, 2011		Dec. 31, 2010
Investment [Line Items]
Total trust accounts on deposit	$ 1,641,805		$ 1,657,441
Total letter of credit facilities	1,700,000		1,700,000
Total letter of credit facilities outstanding	914,089		848,834
Total letter of credit facilities remaining	785,911		851,166
Collateral committed to support the letter of credit facilities	1,062,380		1,121,345
Citibank Europe Plc [Member]
Investment [Line Items]
Total letter of credit facilities	900,000		900,000
Total letter of credit facilities outstanding	752,412		689,851
Total letter of credit facilities remaining	147,588		210,149
Credit Facility [Member]
Investment [Line Items]
Total letter of credit facilities	800,000		800,000
Total letter of credit facilities outstanding	161,677		158,983
Total letter of credit facilities remaining	$ 638,323	[1]	$ 641,017	[1]

[1] Net of any borrowing or repayments under the Unsecured Facility (as defined in Note 8). See Note 8 for further details on the Unsecured Facility.