Long-Term Debt and Credit Facilities (Details 2) (USD $)	3 Months Ended								12 Months Ended										12 Months Ended													12 Months Ended				12 Months Ended				12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 AGUS (Consolidated)	Dec. 31, 2010 AGUS (Consolidated)	Dec. 31, 2009 AGUS (Consolidated)	Dec. 31, 2011 AGUS (Consolidated) 7.0% Senior Notes	Dec. 31, 2010 AGUS (Consolidated) 7.0% Senior Notes	Dec. 31, 2009 AGUS (Consolidated) 7.0% Senior Notes	May 18, 2004 AGUS (Consolidated) 7.0% Senior Notes	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes	Dec. 31, 2010 AGUS (Consolidated) 8.50% Senior Notes	Dec. 31, 2009 AGUS (Consolidated) 8.50% Senior Notes	Dec. 31, 2011 AGUS (Consolidated) Series A Enhanced Junior Subordinated Debentures	Dec. 31, 2010 AGUS (Consolidated) Series A Enhanced Junior Subordinated Debentures	Dec. 31, 2009 AGUS (Consolidated) Series A Enhanced Junior Subordinated Debentures	Dec. 31, 2011 AGMH (Consolidated)	Dec. 31, 2010 AGMH (Consolidated)	Dec. 31, 2009 AGMH (Consolidated)	Dec. 31, 2011 AGMH (Consolidated) 6 7/8% QUIBS	Dec. 31, 2010 AGMH (Consolidated) 6 7/8% QUIBS	Dec. 31, 2009 AGMH (Consolidated) 6 7/8% QUIBS	Dec. 19, 2001 AGMH (Consolidated) 6 7/8% QUIBS	Dec. 31, 2011 AGMH (Consolidated) 6.25% Notes	Dec. 31, 2010 AGMH (Consolidated) 6.25% Notes	Dec. 31, 2009 AGMH (Consolidated) 6.25% Notes	Nov. 26, 2002 AGMH (Consolidated) 6.25% Notes	Dec. 31, 2011 AGMH (Consolidated) 5.60% Notes	Dec. 31, 2010 AGMH (Consolidated) 5.60% Notes	Dec. 31, 2009 AGMH (Consolidated) 5.60% Notes	Jul. 31, 2003 AGMH (Consolidated) 5.60% Notes	Dec. 31, 2011 AGMH (Consolidated) Junior Subordinated Debentures	Dec. 31, 2010 AGMH (Consolidated) Junior Subordinated Debentures	Dec. 31, 2009 AGMH (Consolidated) Junior Subordinated Debentures	Dec. 31, 2011 AGM	Dec. 31, 2010 AGM	Dec. 31, 2009 AGM	Dec. 31, 2011 AGM Notes Payable	Dec. 31, 2010 AGM Notes Payable	Dec. 31, 2009 AGM Notes Payable
Debt
Principal	$ 1,349,600,000				$ 1,371,800,000				$ 1,349,600,000	$ 1,371,800,000		$ 522,500,000	$ 522,500,000		$ 200,000,000	$ 200,000,000			$ 172,500,000	$ 172,500,000		$ 150,000,000	$ 150,000,000		$ 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		$ 97,100,000	$ 119,300,000		$ 97,100,000	$ 119,300,000
Carrying Value	1,038,302,000				1,052,936,000				1,038,302,000	1,052,936,000		519,500,000	518,400,000		197,600,000	197,600,000			172,000,000	171,000,000		149,900,000	149,800,000		415,100,000	407,500,000		67,400,000	67,000,000			136,000,000	135,000,000			53,500,000	53,000,000			158,200,000	152,500,000		103,700,000	127,000,000		103,700,000	127,000,000
Interest rate of debt (as a percent)															7.00%	7.00%		7.00%	8.50%	8.50%								6.875%	6.875%		6.875%	6.25%	6.25%		6.25%	5.60%	5.60%		5.60%	6.40%
Expected Maturity Schedule of Long Term Debt
2012	31,900,000								31,900,000																																		31,900,000
2013	20,100,000								20,100,000																																		20,100,000
2014	186,200,000								186,200,000			172,500,000																															13,700,000
2015	8,900,000								8,900,000																																		8,900,000
2016	5,100,000								5,100,000																																		5,100,000
2017-2036	217,400,000								217,400,000			200,000,000																															17,400,000
2057-2076	450,000,000								450,000,000			150,000,000													300,000,000
Thereafter	430,000,000								430,000,000																430,000,000
Total	1,349,600,000				1,371,800,000				1,349,600,000	1,371,800,000		522,500,000	522,500,000		200,000,000	200,000,000			172,500,000	172,500,000		150,000,000	150,000,000		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		97,100,000	119,300,000		97,100,000	119,300,000
Interest expenses	$ 24,700,000	$ 24,900,000	$ 24,700,000	$ 24,800,000	$ 24,700,000	$ 24,900,000	$ 24,900,000	$ 25,100,000	$ 99,112,000	$ 99,621,000	$ 62,783,000	$ 39,400,000	$ 39,300,000	$ 31,600,000	$ 13,500,000	$ 13,500,000	$ 13,500,000		$ 16,100,000	$ 16,000,000	$ 8,300,000	$ 9,800,000	$ 9,800,000	$ 9,800,000	$ 53,600,000	$ 53,600,000	$ 26,800,000	$ 7,200,000	$ 7,200,000	$ 3,600,000		$ 15,400,000	$ 15,400,000	$ 7,700,000		$ 6,100,000	$ 6,100,000	$ 3,100,000		$ 24,900,000	$ 24,900,000	$ 12,400,000	$ 6,100,000	$ 6,700,000	$ 4,400,000	$ 6,100,000	$ 6,700,000	$ 4,400,000