Income Taxes (Details) (USD $)	1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 30, 2012	Mar. 31, 2012	Mar. 31, 2011	Apr. 01, 2012
Income Taxes
U.S. marginal corporate income tax rate (as a percent)		35.00%
U.K. marginal corporate tax rate (as a percent)	24.00%	26.00%	28.00%	26.00%
Effective Tax Rate Reconciliation
Expected tax provision (benefit) at statutory rates in taxable jurisdictions		$ (198,800,000)	$ 87,700,000
Tax-exempt interest		(15,400,000)	(15,300,000)
Change in liability for uncertain tax positions		600,000	600,000
Other		1,000,000	900,000
Total provision (benefit) for income taxes		(212,595,000)	73,932,000
Effective tax rate (as a percent)		30.60%	34.70%
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income		(695,619,000)	213,188,000
Revenue		(357,406,000)	278,913,000
Period for which cumulative operating income was considered for assessment of realization of deferred tax assets (in years)		3 years
United States
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income		(568,200,000)	250,700,000
Revenue		(280,400,000)	293,700,000
Bermuda.
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income		(127,600,000)	(37,600,000)
Revenue		(77,000,000)	(14,800,000)
UK
Pretax Income and Revenue by Tax Jurisdiction
Pretax Income		200,000	100,000
Revenue		$ 0	$ 100,000