Subsidiary Information - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Total investment portfolio and cash	$ 11,539	$ 11,103
Investment in subsidiaries	0	0
Premiums receivable, net of commissions payable	915	576
Ceded unearned premium reserve	119	206
Deferred acquisition costs	101	106	$ 114	$ 121
Reinsurance recoverable on unpaid losses	44	80
Credit derivative assets	2	13
Deferred tax asset, net	98	497
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	700	876
Dividend receivable from affiliate		0
Other	915	694
Total assets	14,433	14,151
Liabilities and shareholders’ equity
Unearned premium reserve	3,475	3,511
Loss and loss adjustment expense reserve	1,444	1,127
Long-term debt	1,292	1,306
Intercompany payable	0	0
Credit derivative liabilities	271	402
Deferred tax liabilities, net	0	0
Financial guaranty variable interest entities’ liabilities, at fair value	757	958
Dividend payable to affiliate		0
Other	355	343
Total liabilities	7,594	7,647
Effect on shareholders’ equity (decrease) increase	6,839	6,504	$ 6,063	$ 5,758
Noncontrolling interest	0	0
Total shareholders' equity	6,839	6,504
Total liabilities and shareholders’ equity	14,433	14,151
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Assets
Total investment portfolio and cash	36	36
Investment in subsidiaries	6,794	6,164
Premiums receivable, net of 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
Dividend receivable from affiliate		300
Other	26	11
Total assets	6,856	6,511
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
Dividend payable to affiliate		0
Other	17	7
Total liabilities	17	7
Effect on shareholders’ equity (decrease) increase	6,839	6,504
Noncontrolling interest	0	0
Total shareholders' equity	6,839	6,504
Total liabilities and shareholders’ equity	6,856	6,511
Reportable Legal Entities [Member] | AGUS (Issuer) [Member]
Assets
Total investment portfolio and cash	319	384
Investment in subsidiaries	6,126	5,696
Premiums receivable, net of 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	59	16
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividend receivable from affiliate		0
Other	0	78
Total assets	6,504	6,174
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and loss adjustment expense reserve	0	0
Long-term debt	843	843
Intercompany payable	60	70
Credit derivative liabilities	0	0
Deferred tax liabilities, net	0	0
Financial guaranty variable interest entities’ liabilities, at fair value	0	0
Dividend payable to affiliate		300
Other	59	3
Total liabilities	962	1,216
Effect on shareholders’ equity (decrease) increase	5,542	4,958
Noncontrolling interest	0	0
Total shareholders' equity	5,542	4,958
Total liabilities and shareholders’ equity	6,504	6,174
Reportable Legal Entities [Member] | AGMH (Issuer) [Member]
Assets
Total investment portfolio and cash	28	22
Investment in subsidiaries	4,048	3,799
Premiums receivable, net of 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
Dividend receivable from affiliate		0
Other	40	26
Total assets	4,116	3,847
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and loss adjustment expense reserve	0	0
Long-term debt	461	453
Intercompany payable	0	0
Credit derivative liabilities	0	0
Deferred tax liabilities, net	51	88
Financial guaranty variable interest entities’ liabilities, at fair value	0	0
Dividend payable to affiliate		0
Other	20	14
Total liabilities	532	555
Effect on shareholders’ equity (decrease) increase	3,584	3,292
Noncontrolling interest	0	0
Total shareholders' equity	3,584	3,292
Total liabilities and shareholders’ equity	4,116	3,847
Reportable Legal Entities [Member] | Other Entities [Member]
Assets
Total investment portfolio and cash	11,484	11,029
Investment in subsidiaries	216	296
Premiums receivable, net of commissions payable	1,074	699
Ceded unearned premium reserve	1,002	1,099
Deferred acquisition costs	144	156
Reinsurance recoverable on unpaid losses	433	484
Credit derivative assets	39	69
Deferred tax asset, net	93	597
Intercompany receivable	60	70
Financial guaranty variable interest entities’ assets, at fair value	700	876
Dividend receivable from affiliate		0
Other	1,171	801
Total assets	16,416	16,176
Liabilities and shareholders’ equity
Unearned premium reserve	4,423	4,488
Loss and loss adjustment expense reserve	1,793	1,596
Long-term debt	6	10
Intercompany payable	300	300
Credit derivative liabilities	308	458
Deferred tax liabilities, net	0	0
Financial guaranty variable interest entities’ liabilities, at fair value	757	958
Dividend payable to affiliate		0
Other	740	665
Total liabilities	8,327	8,475
Effect on shareholders’ equity (decrease) increase	7,873	7,405
Noncontrolling interest	216	296
Total shareholders' equity	8,089	7,701
Total liabilities and shareholders’ equity	16,416	16,176
Consolidation, Eliminations [Member]
Assets
Total investment portfolio and cash	(328)	(368)
Investment in subsidiaries	(17,184)	(15,955)
Premiums receivable, net of commissions payable	(159)	(123)
Ceded unearned premium reserve	(883)	(893)
Deferred acquisition costs	(43)	(50)
Reinsurance recoverable on unpaid losses	(389)	(404)
Credit derivative assets	(37)	(56)
Deferred tax asset, net	(54)	(116)
Intercompany receivable	(60)	(70)
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividend receivable from affiliate		(300)
Other	(322)	(222)
Total assets	(19,459)	(18,557)
Liabilities and shareholders’ equity
Unearned premium reserve	(948)	(977)
Loss and loss adjustment expense reserve	(349)	(469)
Long-term debt	(18)	0
Intercompany payable	(360)	(370)
Credit derivative liabilities	(37)	(56)
Deferred tax liabilities, net	(51)	(88)
Financial guaranty variable interest entities’ liabilities, at fair value	0	0
Dividend payable to affiliate		(300)
Other	(481)	(346)
Total liabilities	(2,244)	(2,606)
Effect on shareholders’ equity (decrease) increase	(16,999)	(15,655)
Noncontrolling interest	(216)	(296)
Total shareholders' equity	(17,215)	(15,951)
Total liabilities and shareholders’ equity	$ (19,459)	$ (18,557)