Business Changes, Risks, Uncertainties and Accounting Developments (Details 4) (USD $)	Jun. 30, 2012	Dec. 31, 2011	Jun. 01, 2012 Forward Contract [Member]	Jun. 30, 2012 AGM and AGC	Jun. 30, 2012 AGUS Consolidated [Member]	Dec. 31, 2011 AGUS Consolidated [Member]	Jun. 30, 2012 AGUS Consolidated [Member] Senior Notes 8.50 Percent [Member]		Dec. 31, 2011 AGUS Consolidated [Member] Senior Notes 8.50 Percent [Member]		Jun. 01, 2012 AGUS Consolidated [Member] Senior Notes 8.50 Percent [Member] Forward Contract [Member]	Jun. 01, 2012 AGL [Member] Forward Contract [Member]
Debt Instrument [Line Items]
Losses Assumed by Reinsurers under Excess of Loss Reinsurance Facility				$ 435,000,000
Debt Instrument, Face Amount	1,153,900,000	1,349,600,000			350,000,000	522,500,000	0	[1]	172,500,000	[1]	172,500,000
Interest rate of debt (as a percent)							8.50%		8.50%		8.50%
Percent of Remarketed Senior Notes Purchased by AGUS			100.00%
AGL Common Shares Issued for Each Fifty Dollars of Proceeds Received From Remarketing Senior Notes												3.8924
Equity Unit by Which Shares Were Issued For Remarketing and Redemption of Senior Notes												$ 50
Settlement Rate in Common Shares After Anti Dilution Adjustments												3.8685
Common Stock, Shares, Issued	193,956,481	182,235,798										13,428,770

[1] On June 1, 2012, AGUS retired all of the Senior Notes that it purchased in the remarketing. See Note 2, Business Changes, Risks, Uncertainties and Accounting Developments.