Debt and Financing Arrangements (Details) In Thousands, unless otherwise specified	6 Months Ended																	6 Months Ended
	Jun. 30, 2011 USD ($)	Jun. 30, 2011 GBP (£)	Dec. 31, 2010 USD ($)	Dec. 31, 2010 GBP (£)	Jun. 30, 2011 Maximum [Member]	Jun. 30, 2011 Maximum [Member] Commitments to Extend Credit [Member] USD ($)	Jun. 30, 2011 Minimum [Member]	Jun. 30, 2011 Minimum [Member] Commitments to Extend Credit [Member] USD ($)	Jun. 30, 2011 Lenders One Thru Four [Member] Commitments to Extend Credit [Member] USD ($)	Jun. 30, 2011 Lenders Five Thru Eight [Member] Commitments to Extend Credit [Member] USD ($)	Jun. 30, 2011 Lenders Nine Thru Twelve [Member] Commitments to Extend Credit [Member] USD ($)	Jun. 30, 2011 Thirteenth Lender [Member] Commitments to Extend Credit [Member] USD ($)	Jun. 30, 2011 Secured Debt [Member] USD ($)	Jun. 30, 2011 Unsecured Debt [Member] USD ($)	Jun. 30, 2011 Five Year Senior Credit Facility [Member] USD ($)	Jun. 30, 2011 Senior Notes [Member] 5.50% Senior Notes [Member]	Jun. 30, 2011 Senior Notes [Member] 7.50% Senior Notes [Member]	Jun. 30, 2011 5.50% Senior Notes [Member] USD ($)	Jun. 30, 2011 7.50% Senior Notes [Member] USD ($)	Jun. 30, 2011 Citibank Europe Plc [Member] USD ($)
Senior notes																		$ 300,000	$ 500,000
Senior notes, interest rate																5.50%	7.50%	5.50%	7.50%
Underwriters price as a percentage of principal amount																		98.89%	99.71%
Effective yield to investors																		5.56%	7.54%
Letter of Credit	871,437		848,834			87,500		20,000	87,500	62,500	45,000	20,000	400,000	400,000	800,000					900,000
Credit facility has options to increase the aggregate commitments	200,000
Ratio of indebtedness to net capital					1.0		0.35
Line of credit facility having varying commitments	$ 108,258	£ 67,400	$ 82,838	£ 53,700
Senior notes due																		Nov. 10, 2020	Aug. 01, 2016