Investments (Schedule of Company's Trust Accounts on Deposits, Outstanding and Remaining Letter of Credit Facilities, Collateral Committed to Support the Letter of Credit Facilities) (Details) (USD $) In Thousands	Jun. 30, 2011		Dec. 31, 2010
Total trust accounts on deposit	$ 1,608,143		$ 1,657,441
Total letter of credit facilities	1,700,000		1,700,000
Total letter of credit facilities outstanding	871,437		848,834
Total letter of credit facilities remaining	828,563		851,166
Collateral committed to support the letter of credit facilities	1,054,206		1,121,345
Citibank Europe Plc [Member]
Total letter of credit facilities	900,000		900,000
Total letter of credit facilities outstanding	712,454		689,851
Total letter of credit facilities remaining	187,546		210,149
Credit Facility [Member]
Total letter of credit facilities	800,000		800,000
Total letter of credit facilities outstanding	158,983		158,983
Total letter of credit facilities remaining	$ 641,017	[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.