Business Changes, Risks, Uncertainties and Accounting Developments (Details 4) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Jan. 12, 2012 AGM and AGC	Dec. 31, 2012 AGUS Consolidated	Dec. 31, 2011 AGUS Consolidated	Jun. 01, 2012 AGUS Consolidated Forward Contracts	Dec. 31, 2012 AGUS Consolidated Senior Notes 8.50 Percent [Member]	Dec. 31, 2011 AGUS Consolidated Senior Notes 8.50 Percent [Member]	Jun. 01, 2012 AGUS Consolidated Senior Notes 8.50 Percent [Member] Forward Contracts	Jun. 01, 2012 AGL Forward Contracts
Debt Instrument [Line Items]
Losses Assumed by Reinsurers under Excess of Loss Reinsurance Facility			$ 435,000,000
Debt Instrument, Face Amount	1,141,000,000	1,350,000,000		350,000,000	523,000,000		0	173,000,000	173,000,000
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	194,003,297	182,235,798								13,428,770