Income Taxes (Details) (USD $)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Income Taxes
U.S. marginal corporate income tax rate (as a percent)				35.00%
U.K. marginal corporate tax rate (as a percent)	26.00%	28.00%		26.50%
Effective Tax Rate Reconciliation
Expected tax provision (benefit) at statutory rates in taxable jurisdictions	$ (41,200,000)		$ 104,100,000	$ 39,300,000	$ 231,700,000
Tax-exempt interest	(16,300,000)		(14,200,000)	(31,600,000)	(28,300,000)
Change in liability for uncertain tax positions	500,000		600,000	1,100,000	1,100,000
Other	1,100,000		500,000	2,000,000	1,400,000
Total provision (benefit) for income taxes	(55,966,000)		90,966,000	10,712,000	205,911,000	86,600,000	36,900,000	43,400,000
Net deferred tax assets	1,012,011,000			1,012,011,000		1,223,958,000	1,158,200,000
Effective tax rate (as a percent)	49.30%		30.90%	13.70%	28.20%
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	(113,633,000)		294,504,000	78,452,000	731,424,000	635,500,000	132,900,000	112,300,000
Revenue	102,012,000		447,685,000	356,065,000	1,114,967,000	1,401,300,000	917,100,000	553,200,000
United States
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	(117,700,000)		283,900,000	112,200,000	648,600,000
Revenue	61,400,000		391,100,000	330,200,000	923,100,000
Bermuda.
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	4,000,000		10,100,000	(33,900,000)	82,200,000
Revenue	40,600,000		56,600,000	25,800,000	191,900,000
UK
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income	100,000		500,000	200,000	600,000
Revenue				$ 100,000