Long-Term Debt and Credit Facilities (Details) (USD $)	1 Months Ended	12 Months Ended	1 Months Ended				1 Months Ended	12 Months Ended											1 Months Ended	12 Months Ended		12 Months Ended		1 Months Ended				1 Months Ended				1 Months Ended			1 Months Ended	12 Months Ended		12 Months Ended
	Jun. 30, 2009	Dec. 31, 2009	May 31, 2004 AGUS (Consolidated) 7.0% Senior Notes	Dec. 31, 2011 AGUS (Consolidated) 7.0% Senior Notes	Dec. 31, 2010 AGUS (Consolidated) 7.0% Senior Notes	May 18, 2004 AGUS (Consolidated) 7.0% Senior Notes	Jun. 30, 2009 AGUS (Consolidated) 8.50% Senior Notes	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes	Dec. 31, 2010 AGUS (Consolidated) 8.50% Senior Notes	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes Applicable market value equal to or greater than $12.93	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes Applicable market value less than or equal to $11.00	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes Applicable market value between $11.00 and $12.93	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes Minimum	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes Minimum Applicable market value equal to or greater than $12.93	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes Minimum Applicable market value between $11.00 and $12.93	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes Maximum	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes Maximum Applicable market value less than or equal to $11.00	Dec. 31, 2011 AGUS (Consolidated) 8.50% Senior Notes Maximum Applicable market value between $11.00 and $12.93	Dec. 31, 2006 AGUS (Consolidated) Series A Enhanced Junior Subordinated Debentures	Dec. 31, 2016 AGUS (Consolidated) Series A Enhanced Junior Subordinated Debentures	Dec. 31, 2011 AGUS (Consolidated) Series A Enhanced Junior Subordinated Debentures	Dec. 31, 2011 AGUS (Consolidated) Series A Enhanced Junior Subordinated Debentures Minimum Payment	Dec. 31, 2011 AGUS (Consolidated) Series A Enhanced Junior Subordinated Debentures Maximum	Dec. 31, 2001 AGMH (Consolidated) 6 7/8% QUIBS	Dec. 31, 2011 AGMH (Consolidated) 6 7/8% QUIBS	Dec. 31, 2010 AGMH (Consolidated) 6 7/8% QUIBS	Dec. 19, 2001 AGMH (Consolidated) 6 7/8% QUIBS	Nov. 30, 2002 AGMH (Consolidated) 6.25% Notes	Dec. 31, 2011 AGMH (Consolidated) 6.25% Notes	Dec. 31, 2010 AGMH (Consolidated) 6.25% Notes	Nov. 26, 2002 AGMH (Consolidated) 6.25% Notes	Jul. 31, 2003 AGMH (Consolidated) 5.60% Notes	Dec. 31, 2011 AGMH (Consolidated) 5.60% Notes	Dec. 31, 2010 AGMH (Consolidated) 5.60% Notes	Nov. 30, 2006 AGMH (Consolidated) Junior Subordinated Debentures	Dec. 31, 2036 AGMH (Consolidated) Junior Subordinated Debentures	Dec. 31, 2011 AGMH (Consolidated) Junior Subordinated Debentures	Dec. 31, 2011 AGMH (Consolidated) Junior Subordinated Debentures Minimum Payment	Dec. 31, 2011 AGMH (Consolidated) Junior Subordinated Debentures Maximum time	Jul. 01, 2009 AGMH (Consolidated) Notes Payable
Debt
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%
Debt issued			$ 200,000,000																$ 150,000,000					$ 100,000,000				$ 230,000,000				$ 100,000,000			$ 300,000,000
Net proceeds from issuance of debt			197,300,000																149,700,000
Effective interest rate of debt (as a percent)				6.40%
Number of equity units issued (in shares)							3,450,000
Net proceeds from issuance of equity units	170,800,000	167,972,000					166,800,000
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
Number of common shares to be issued under forward purchase contract										3.8685	4.5455		3.8685			4.5455
Price for which shares are required to be purchased under purchase contract								50
Number of trading days over which average closing price of common shares is to be considered for determining actual number of shares issuable for each equity unit								20 days
Number of trading days prior to June 1, 2012 by which 20-trading days for calculating average closing price should end								3 days
Average closing price per share														$ 12.93	$ 11.00		$ 11.00	$ 12.93
Numerator for the calculation of the number of shares issuable under the forward purchase contract												50
Number of designated business days during which remarketing is required								3 days
Period required between interest rate reset date and new redemption date for mandatory remarketing (in years)													2 years
Redemption price of each unit of debt instrument								1,000
Fixed rate of interest (as a percent)																					6.40%
Interest rate, description																				3 months LIBOR																one-month LIBOR
Interest rate, added to base rate (as a percent)																				2.38%																2.215%
Number of times interest payment may be deferred																						1																1
Number of consecutive periods for which interest payments may be deferred																						1																1
Period for which interest payment may be deferred (in years)																							10 years																10 years
Number of times repayment date may be extended																																							4
Period for each increment of repayment date extension (in years)																																					5 years
Period prior to final repayment date before which debt cannot be repaid, redeemed, repurchased or defeased (in years)																																					20 years
Fair value of debt on Acquisition Date																																								164,400,000
Premium on debt instrument																																								$ 9,500,000