Long-Term Debt and Credit Facilities (Details) (USD $)	6 Months Ended									1 Months Ended			1 Months Ended			1 Months Ended			11 Months Ended	12 Months Ended				5 Months Ended			6 Months Ended			12 Months Ended							12 Months Ended
	Jun. 30, 2012		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 AGMH	Dec. 31, 2011 AGMH	Dec. 31, 2012 AGMH Enhanced Junior Subordinated Debentures [Member]	Dec. 31, 2012 AGMH Consolidated	Dec. 31, 2011 AGMH Consolidated	Dec. 19, 2001 AGMH Consolidated 6 7/8% QUIBS [Member]	Dec. 31, 2012 AGMH Consolidated 6 7/8% QUIBS [Member]	Dec. 31, 2011 AGMH Consolidated 6 7/8% QUIBS [Member]	Nov. 26, 2002 AGMH Consolidated 6.25% Notes [Member]	Dec. 31, 2012 AGMH Consolidated 6.25% Notes [Member]	Dec. 31, 2011 AGMH Consolidated 6.25% Notes [Member]	Jul. 31, 2003 AGMH Consolidated 5.60% Notes [Member]	Dec. 31, 2012 AGMH Consolidated 5.60% Notes [Member]	Dec. 31, 2011 AGMH Consolidated 5.60% Notes [Member]	Nov. 30, 2006 AGMH Consolidated Junior Subordinated Debentures [Member]	Dec. 31, 2012 AGMH Consolidated Junior Subordinated Debentures [Member]	Dec. 31, 2011 AGMH Consolidated Junior Subordinated Debentures [Member]	Dec. 31, 2012 AGUS Consolidated	Dec. 31, 2011 AGUS Consolidated	May 18, 2004 AGUS Consolidated 7.0% Senior Notes [Member]	Dec. 31, 2012 AGUS Consolidated 7.0% Senior Notes [Member]	Dec. 31, 2011 AGUS Consolidated 7.0% Senior Notes [Member]	Jun. 30, 2009 AGUS Consolidated 8.50% Senior Notes [Member]	Dec. 31, 2012 AGUS Consolidated 8.50% Senior Notes [Member]	Dec. 31, 2011 AGUS Consolidated 8.50% Senior Notes [Member]	Dec. 31, 2012 AGUS Consolidated Enhanced Junior Subordinated Debentures [Member]	Dec. 31, 2006 AGUS Consolidated Enhanced Junior Subordinated Debentures [Member]	Dec. 31, 2011 AGUS Consolidated Enhanced Junior Subordinated Debentures [Member]	Dec. 31, 2012 AGM	Dec. 31, 2011 AGM	Dec. 31, 2012 AGM Notes Payable	Dec. 31, 2011 AGM Notes Payable	Dec. 31, 2012 Minimum AGUS Consolidated Enhanced Junior Subordinated Debentures [Member]	Dec. 31, 2012 Maximum AGMH Consolidated Junior Subordinated Debentures [Member]	Jun. 01, 2012 Forward Contracts AGUS Consolidated 8.50% Senior Notes [Member]	Jun. 01, 2012 Forward Contracts AGL
Debt
Interest rate, descripton																				One month LIBOR										3 month LIBOR
Debt issued										$ 100,000,000			$ 230,000,000			$ 100,000,000			$ 300,000,000					$ 200,000,000							$ 150,000,000
Number of times repayment date may be extended																																						4
Period for Increments of Repayment Date Extension																																						5 years
Net proceeds from issuance of debt																								197,000,000			167,000,000
Debt Instrument, Interest Rate, Effective Percentage																								640.00%
Percentage of undivided beneficial ownership interest in debt principal per equity unit																												5.00%
Principal amount of debt for percentage of undivided beneficial ownership interest per equity unit																												1,000
Price for which Shares are Issuable under Forward Purchase Contract for Each Equity Unit																												50
Number of equity units issued (in shares)																											3,450,000
Principal			1,141,000,000	1,350,000,000				730,000,000	730,000,000		100,000,000	100,000,000		230,000,000	230,000,000		100,000,000	100,000,000		300,000,000	300,000,000	350,000,000	523,000,000		200,000,000	200,000,000		0	173,000,000	150,000,000		150,000,000	61,000,000	97,000,000	61,000,000	97,000,000			173,000,000
Carrying Value			$ 836,000,000	$ 1,038,000,000	$ 423,000,000	$ 415,000,000		$ 423,000,000	$ 415,000,000		$ 68,000,000	$ 67,000,000		$ 137,000,000	$ 136,000,000		$ 54,000,000	$ 54,000,000		$ 164,000,000	$ 158,000,000	$ 347,000,000	$ 519,000,000		$ 197,000,000	$ 197,000,000		$ 0	$ 172,000,000	$ 150,000,000		$ 150,000,000	$ 66,000,000	$ 104,000,000	$ 66,000,000	$ 104,000,000
Interest rate of debt (as a percent)											6.875%	6.875%		6.25%	6.25%		5.60%	5.60%		6.40%				700.00%	7.00%	7.00%		8.50%	8.50%										8.50%
AGL Common Shares Issued for Each Fifty Dollars of Proceeds Received From Remarketing Senior Notes																																								3.8924
Settlement Rate in Common Shares After Anti Dilution Adjustments																																								3.8685
Common Stock, Shares, Issued			194,003,297	182,235,798																																				13,428,770
Price per share of stock issued (per share)	$ 12.85	[1]
Interest rate, added to base rate (as a percent)							2.215%																							2.38%
Number of times interest payment may be deferred																																					1
Number of consecutive periods for which interest payments may be deferred																																					1
Period for which interest payment may be deferred (in years)																																					10 years
Period prior to final repayment date before which debt cannot be repaid, redeemed, repurchased or defeased (in years)																																					20 years

[1] Relates to the settlement of forward purchase contracts. See Note 2, Business Changes, Risks, Uncertainties and Accounting Developments.