Consolidation of Variable Interest Entities (Details) (USD $)	3 Months Ended				6 Months Ended				12 Months Ended
	Jun. 30, 2012 number contract		Jun. 30, 2011 Transaction Entity		Jun. 30, 2012 number contract		Jun. 30, 2011		Dec. 31, 2011 number
Consolidated VIEs
Number of VIE that did not require consolidation	1,200				1,200
Value Net Fair Value Gains and Losses on Fg Vies Are Expected to Reverse to on Maurity of Vie Debt Except Claim Payments Paid by Agc or Agm Under Financial Guaranty Insurance Contracts	$ 0				$ 0
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Variable Interest Entity, Number of Entities, With Recourse	34				34				33
Variable Interest Entity, Number of Entities, Without Recourse	0				0				0
Number of FG VIEs	34				34				33
Assets	2,725,979,000				2,725,979,000				2,819,077,000
Liabilities	3,281,300,000				3,281,300,000				3,458,400,000
Total with recourse , Assets	1,683,700,000				1,683,700,000				1,757,600,000
Without recourse, Assets	1,042,300,000				1,042,300,000				1,061,500,000
Total unpaid principal balance for the VIEs' assets that were over 90 days or more past due	1,001,300,000				1,001,300,000
Number of days VIEs' assets were past due (in days)					90 days
Difference between the aggregate unpaid principal and aggregate fair value of the VIEs' Assets	2,994,700,000				2,994,700,000
Change in the instrument specific credit risk of the VIEs' assets	84,500,000		861,900,000		170,400,000		478,700,000
Difference between the aggregate unpaid principal and aggregate fair value of the VIEs' liabilities	2,411,300,000				2,411,300,000
Contractual Maturity
2012	0				0
2013	0				0
2014	90,800,000				90,800,000
2015	0				0
2016	0				0
Thereafter	3,438,600,000				3,438,600,000
Gross par outstanding for FG VIEs liabilities with recourse	3,529,400,000				3,529,400,000				3,796,400,000
Net earned premiums	(15,500,000)	[1]	(18,300,000)	[1]	(32,500,000)	[1]	(37,400,000)	[1]
Net investment income	(2,600,000)	[1]	(400,000)	[1]	(5,800,000)	[1]	(700,000)	[1]
Net realized investment gains (losses)	2,100,000	[1]	200,000	[1]	3,500,000	[1]	500,000	[1]
Net change in fair value of financial guaranty variable interest entities:
Fair value gains (losses) on financial guaranty variable interest entities	172,356,000		(174,286,000)		135,754,000		(54,685,000)
Loss and LAE	(500,000)	[1]	16,900,000	[1]	2,700,000	[1]	67,600,000	[1]
Total pre-tax effect on net income	155,900,000	[1]	(175,900,000)	[1]	103,700,000	[1]	(24,700,000)	[1]
Less: tax provision (benefit)	54,600,000	[1]	(61,600,000)	[1]	36,300,000	[1]	(8,600,000)	[1]
Total effect on net income (loss)	101,300,000	[1]	(114,300,000)	[1]	67,400,000	[1]	(16,100,000)	[1]
Total (decrease) increase on shareholders' equity					(339,300,000)				(405,200,000)
Number of Variable Interest Entities Consolidated			8
Number of Transactions That the Fair Value of Underlying Collateral Depreciated and Related Wrapped Senior Bonds Value Unchanged			2
AGC R&W Benefits From Deutsche Bank Settlement [Member]
Net change in fair value of financial guaranty variable interest entities:
Fair value gains (losses) on financial guaranty variable interest entities	160,900,000
HELOCs
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Variable Interest Entity, Number of Entities, With Recourse	8				8				8
Assets	550,200,000				550,200,000				572,900,000
Liabilities	863,600,000				863,600,000				907,900,000
Alt-A first lien
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Variable Interest Entity, Number of Entities, With Recourse	3				3				3
Assets	114,800,000				114,800,000				118,000,000
Liabilities	101,500,000				101,500,000				106,100,000
Option ARM
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Variable Interest Entity, Number of Entities, With Recourse	2				2				2
Assets	40,100,000				40,100,000				49,700,000
Liabilities	199,500,000				199,500,000				244,700,000
Subprime
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Variable Interest Entity, Number of Entities, With Recourse	7				7				5
Assets	391,300,000				391,300,000				386,800,000
Liabilities	493,600,000				493,600,000				472,700,000
CES
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Variable Interest Entity, Number of Entities, With Recourse	10				10				10
Assets	205,700,000				205,700,000				184,600,000
Liabilities	199,200,000				199,200,000				219,900,000
Automobile loans
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Variable Interest Entity, Number of Entities, With Recourse	3				3				4
Assets	91,400,000				91,400,000				155,800,000
Liabilities	91,400,000				91,400,000				155,800,000
Life insurance
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Variable Interest Entity, Number of Entities, With Recourse	1				1				1
Assets	290,200,000				290,200,000				289,800,000
Liabilities	$ 290,200,000				$ 290,200,000				$ 289,800,000

[1] Includes the effect of eliminating insurance balances related to the financial guaranty insurance contracts.