Subsidiary Information (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012	Sep. 30, 2012		Dec. 31, 2011
Assets
Total investment portfolio and cash	$ 10,866		$ 11,223
Investment in subsidiaries	0		0
Premiums receivable, net of ceding commissions payable	906	[1]	1,005	944	[1]	1,003
Ceded unearned premium reserve	480		561
Deferred acquisition costs	125		116
Reinsurance recoverable on unpaid losses	59		58
Credit derivative assets	106		141
Deferred tax asset, net	767		721
Intercompany receivable	0		0
Financial guaranty variable interest entities’ assets, at fair value	2,515		2,688
Other	530		729
Total assets	16,354		17,242
Liabilities and shareholders’ equity
Unearned premium reserve	4,676		5,207
Loss and loss adjustment expense reserve	601		601
Long-term debt	819		836
Intercompany payable	0		0
Credit derivative liabilities	2,027		1,934
Deferred Tax Liabilities, Net	0
Financial guaranty variable interest entities’ liabilities, at fair value	2,875		3,141
Other	522		529
Total liabilities	11,520		12,248
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	4,834		4,994
Noncontrolling interest	0
TOTAL SHAREHOLDERS' EQUITY	4,834
Total liabilities and shareholders’ equity	16,354		17,242
Assured Guaranty Ltd. (Parent)
Assets
Total investment portfolio and cash	36		245
Investment in subsidiaries	4,779		4,734
Premiums receivable, net of ceding commissions payable	0		0
Ceded unearned premium reserve	0		0
Deferred acquisition costs	0		0
Reinsurance recoverable on unpaid losses	0		0
Credit derivative assets	0		0
Deferred tax asset, net	0		0
Intercompany receivable	0		0
Financial guaranty variable interest entities’ assets, at fair value	0		0
Other	26		23
Total assets	4,841		5,002
Liabilities and shareholders’ equity
Unearned premium reserve	0		0
Loss and loss adjustment expense reserve	0		0
Long-term debt	0		0
Intercompany payable	0		0
Credit derivative liabilities	0		0
Deferred Tax Liabilities, Net	0
Financial guaranty variable interest entities’ liabilities, at fair value	0		0
Other	7		8
Total liabilities	7		8
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	4,834		4,994
Noncontrolling interest	0
TOTAL SHAREHOLDERS' EQUITY	4,834
Total liabilities and shareholders’ equity	4,841		5,002
AGUS (Issuer)
Assets
Total investment portfolio and cash	133		15
Investment in subsidiaries	4,030		3,958
Premiums receivable, net of ceding commissions payable	0		0
Ceded unearned premium reserve	0		0
Deferred acquisition costs	0		0
Reinsurance recoverable on unpaid losses	0		0
Credit derivative assets	0		0
Deferred tax asset, net	76		48
Intercompany receivable	0		0
Financial guaranty variable interest entities’ assets, at fair value	0		0
Other	2		29
Total assets	4,241		4,050
Liabilities and shareholders’ equity
Unearned premium reserve	0		0
Loss and loss adjustment expense reserve	0		0
Long-term debt	348		347
Intercompany payable	90		173
Credit derivative liabilities	0		0
Deferred Tax Liabilities, Net	0
Financial guaranty variable interest entities’ liabilities, at fair value	0		0
Other	7		6
Total liabilities	445		526
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	3,796		3,524
Noncontrolling interest	0
TOTAL SHAREHOLDERS' EQUITY	3,796
Total liabilities and shareholders’ equity	4,241		4,050
AGMH (Issuer)
Assets
Total investment portfolio and cash	49		30
Investment in subsidiaries	3,497		3,225
Premiums receivable, net of ceding commissions payable	0		0
Ceded unearned premium reserve	0		0
Deferred acquisition costs	0		0
Reinsurance recoverable on unpaid losses	0		0
Credit derivative assets	0		0
Deferred tax asset, net	0		(94)
Intercompany receivable	0		0
Financial guaranty variable interest entities’ assets, at fair value	0		0
Other	39		26
Total assets	3,585		3,187
Liabilities and shareholders’ equity
Unearned premium reserve	0		0
Loss and loss adjustment expense reserve	0		0
Long-term debt	428		423
Intercompany payable	0		0
Credit derivative liabilities	0		0
Deferred Tax Liabilities, Net	91
Financial guaranty variable interest entities’ liabilities, at fair value	0		0
Other	21		15
Total liabilities	540		438
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	3,045		2,749
Noncontrolling interest	0
TOTAL SHAREHOLDERS' EQUITY	3,045
Total liabilities and shareholders’ equity	3,585		3,187
Other Subsidiaries
Assets
Total investment portfolio and cash	10,948		11,233
Investment in subsidiaries	280		3,524
Premiums receivable, net of ceding commissions payable	1,047		1,147
Ceded unearned premium reserve	1,646		1,550
Deferred acquisition costs	204		190
Reinsurance recoverable on unpaid losses	196		223
Credit derivative assets	530		553
Deferred tax asset, net	805		789
Intercompany receivable	90		173
Financial guaranty variable interest entities’ assets, at fair value	2,515		2,688
Other	716		816
Total assets	18,977		22,886
Liabilities and shareholders’ equity
Unearned premium reserve	5,820		6,168
Loss and loss adjustment expense reserve	751		778
Long-term debt	43		66
Intercompany payable	300		300
Credit derivative liabilities	2,451		2,346
Deferred Tax Liabilities, Net	0
Financial guaranty variable interest entities’ liabilities, at fair value	2,875		3,141
Other	877		803
Total liabilities	13,117		13,602
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	5,580		9,284
Noncontrolling interest	280
TOTAL SHAREHOLDERS' EQUITY	5,860
Total liabilities and shareholders’ equity	18,977		22,886
Consolidating Adjustments
Assets
Total investment portfolio and cash	(300)		(300)
Investment in subsidiaries	(12,586)		(15,441)
Premiums receivable, net of ceding commissions payable	(141)		(142)
Ceded unearned premium reserve	(1,166)		(989)
Deferred acquisition costs	(79)		(74)
Reinsurance recoverable on unpaid losses	(137)		(165)
Credit derivative assets	(424)		(412)
Deferred tax asset, net	(114)		(22)
Intercompany receivable	(90)		(173)
Financial guaranty variable interest entities’ assets, at fair value	0		0
Other	(253)		(165)
Total assets	(15,290)		(17,883)
Liabilities and shareholders’ equity
Unearned premium reserve	(1,144)		(961)
Loss and loss adjustment expense reserve	(150)		(177)
Long-term debt	0		0
Intercompany payable	(390)		(473)
Credit derivative liabilities	(424)		(412)
Deferred Tax Liabilities, Net	(91)
Financial guaranty variable interest entities’ liabilities, at fair value	0		0
Other	(390)		(303)
Total liabilities	(2,589)		(2,326)
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	(12,421)		(15,557)
Noncontrolling interest	(280)
TOTAL SHAREHOLDERS' EQUITY	(12,701)
Total liabilities and shareholders’ equity	$ (15,290)		$ (17,883)

[1] Excludes $19 million and $30 million as of September 30, 2013 and September 30, 2012, respectively, related to consolidated FG VIEs.