Consolidation of Variable Interest Entities (Details) (USD $)	3 Months Ended			12 Months Ended
	Mar. 31, 2013 Entity		Mar. 31, 2012	Dec. 31, 2012 Entity		Dec. 31, 2011 Entity
Consolidated VIEs
Number of VIE that did not require consolidation	1,100
Net fair value gains and losses on FG VIEs are expected to reverse to zero at maturity of the VIE debt	$ 0
Variable Interest Entity, Number of Entities to be Consolidated	11	[1]		2	[1]
Variable Interest Entity Consolidation Gain or Loss	500,000		6,000,000
Variable Interest Entity Number Matured	(1)			(2)
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Number of FG VIEs	43			33		33
Number of days VIEs' assets were past due (in days)	90 days
Total unpaid principal balance for the VIEs' assets that were over 90 days or more past due	862,000,000
Difference between the aggregate unpaid principal and aggregate fair value of the VIEs' Assets	2,432,000,000
Change in the instrument specific credit risk of the VIEs' assets	71,000,000
Difference between the aggregate unpaid principal and aggregate fair value of the VIEs' liabilities	1,977,000,000
VIE's with recourse	43			33
VIE's without recourse	0			0
Assets	2,813,000,000			2,688,000,000
Liabilities	3,178,000,000			3,141,000,000
Total with recourse , Assets	1,672,000,000			1,601,000,000
Financial guaranty variable interest entities’ liabilities with recourse, at fair value	2,071,000,000			2,090,000,000
Without recourse, Assets	1,141,000,000			1,087,000,000
Financial guaranty variable interest entities’ liabilities without recourse, at fair value	1,107,000,000			1,051,000,000
Contractual Maturity
2014	17,000,000
Gross unpaid principal for FG VIEs’ liabilities with recourse (1)	2,707,000,000	[2]		2,808,000,000	[2]
Net change in fair value of financial guaranty variable interest entities:
Net earned premiums	(18,000,000)		(17,000,000)
Net investment income	(3,000,000)		(3,000,000)
Net realized investment gains (losses)	1,000,000		1,000,000
Fair value gains (losses) on FG VIEs	70,000,000		(41,000,000)
Loss and LAE	(7,000,000)		8,000,000
Total pre-tax effect on net income	43,000,000		(52,000,000)
Less: tax provision (benefit)	15,000,000		(18,000,000)
Total effect on net income (loss)	28,000,000		(34,000,000)
Total (decrease) increase on shareholders' equity	(322,000,000)			(348,000,000)
AGC R&W Benefits From Deutsche Bank Settlement
Net change in fair value of financial guaranty variable interest entities:
Fair value gains (losses) on FG VIEs	64,000,000
Number of transactions that the fair value of underlying collateral depreciated and related wrapped senior bonds value unchanged	2
HELOCs
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
VIE's with recourse	8			8
Assets	582,000,000			525,000,000
Liabilities	764,000,000			786,000,000
Alt-A first lien
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
VIE's with recourse	5			5
Assets	178,000,000			177,000,000
Liabilities	168,000,000			162,000,000
First mortgage, option adjustable rate mortgage
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
VIE's with recourse	2			2
Assets	41,000,000			42,000,000
Liabilities	158,000,000			170,000,000
First mortgage, subprime
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
VIE's with recourse	18			7
Assets	415,000,000			399,000,000
Liabilities	502,000,000			493,000,000
Close ended second lien
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
VIE's with recourse	8			8
Assets	103,000,000			108,000,000
Liabilities	126,000,000			129,000,000
Automobile loans
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
VIE's with recourse	1			2
Assets	17,000,000			39,000,000
Liabilities	17,000,000			39,000,000
Life insurance
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
VIE's with recourse	1			1
Assets	336,000,000			311,000,000
Liabilities	$ 336,000,000			$ 311,000,000

[1] (1)Net loss on consolidation was $0.5 million in First Quarter 2013 and $6 million in 2012 and recorded in “fair value gains (losses) on FG VIEs” in the consolidated statement of operations.

[2] FG VIE liabilities with recourse will mature at various dates ranging from 2018 to 2047, except for $17 million maturing in 2014.