Subsidiary Information - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets
Total investment portfolio and cash	$ 11,265	$ 11,539
Investment in subsidiaries	0	0
Premiums receivable, net of commissions payable	944	915
Ceded unearned premium reserve	122	119
Deferred acquisition costs	100	101
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	651	700
Dividend receivable from affiliate	0
Other	937	1,059
Total assets	14,019	14,433
Liabilities and shareholders’ equity
Unearned premium reserve	3,395	3,475
Loss and loss adjustment expense reserve	1,299	1,444
Long-term debt	1,281	1,292
Intercompany payable	0	0
Credit derivative liabilities	237	271
Financial guaranty variable interest entities’ liabilities, at fair value	708	757
Dividend payable to affiliate	0
Other	315	355
Total liabilities	7,235	7,594
Effect on shareholders' equity (decrease) increase	6,784	6,839
Noncontrolling interest	0	0
Total shareholders' equity	6,784	6,839
Total liabilities and shareholders’ equity	14,019	14,433
AGUS [Member]
Liabilities and shareholders’ equity
Long-term debt	843	843
AGMH [Member]
Liabilities and shareholders’ equity
Long-term debt	463	461
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Assets
Total investment portfolio and cash	35	36
Investment in subsidiaries	6,657	6,794
Premiums receivable, net of commissions payable	0	0
Ceded unearned premium reserve	0	0
Deferred acquisition costs	0	0
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividend receivable from affiliate	95
Other	21	26
Total assets	6,808	6,856
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
Financial guaranty variable interest entities’ liabilities, at fair value	0	0
Dividend payable to affiliate	0
Other	24	17
Total liabilities	24	17
Effect on shareholders' equity (decrease) increase	6,784	6,839
Noncontrolling interest	0	0
Total shareholders' equity	6,784	6,839
Total liabilities and shareholders’ equity	6,808	6,856
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGUS [Member]
Assets
Total investment portfolio and cash	528	319
Investment in subsidiaries	5,902	6,126
Premiums receivable, net of commissions payable	0	0
Ceded unearned premium reserve	0	0
Deferred acquisition costs	0	0
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividend receivable from affiliate	0
Other	31	59
Total assets	6,461	6,504
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	60
Credit derivative liabilities	0	0
Financial guaranty variable interest entities’ liabilities, at fair value	0	0
Dividend payable to affiliate	95
Other	27	59
Total liabilities	1,025	962
Effect on shareholders' equity (decrease) increase	5,436	5,542
Noncontrolling interest	0	0
Total shareholders' equity	5,436	5,542
Total liabilities and shareholders’ equity	6,461	6,504
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGMH [Member]
Assets
Total investment portfolio and cash	40	28
Investment in subsidiaries	4,032	4,048
Premiums receivable, net of commissions payable	0	0
Ceded unearned premium reserve	0	0
Deferred acquisition costs	0	0
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividend receivable from affiliate	0
Other	46	40
Total assets	4,118	4,116
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and loss adjustment expense reserve	0	0
Long-term debt	463	461
Intercompany payable	0	0
Credit derivative liabilities	0	0
Financial guaranty variable interest entities’ liabilities, at fair value	0	0
Dividend payable to affiliate	0
Other	75	71
Total liabilities	538	532
Effect on shareholders' equity (decrease) increase	3,580	3,584
Noncontrolling interest	0	0
Total shareholders' equity	3,580	3,584
Total liabilities and shareholders’ equity	4,118	4,116
Reportable Legal Entities [Member] | Other Entities [Member]
Assets
Total investment portfolio and cash	11,010	11,484
Investment in subsidiaries	220	216
Premiums receivable, net of commissions payable	1,104	1,074
Ceded unearned premium reserve	994	1,002
Deferred acquisition costs	141	144
Intercompany receivable	60	60
Financial guaranty variable interest entities’ assets, at fair value	651	700
Dividend receivable from affiliate	0
Other	1,507	1,736
Total assets	15,687	16,416
Liabilities and shareholders’ equity
Unearned premium reserve	4,324	4,423
Loss and loss adjustment expense reserve	1,620	1,793
Long-term debt	6	6
Intercompany payable	300	300
Credit derivative liabilities	269	308
Financial guaranty variable interest entities’ liabilities, at fair value	708	757
Dividend payable to affiliate	0
Other	628	740
Total liabilities	7,855	8,327
Effect on shareholders' equity (decrease) increase	7,612	7,873
Noncontrolling interest	220	216
Total shareholders' equity	7,832	8,089
Total liabilities and shareholders’ equity	15,687	16,416
Consolidation, Eliminations [Member]
Assets
Total investment portfolio and cash	(348)	(328)
Investment in subsidiaries	(16,811)	(17,184)
Premiums receivable, net of commissions payable	(160)	(159)
Ceded unearned premium reserve	(872)	(883)
Deferred acquisition costs	(41)	(43)
Intercompany receivable	(60)	(60)
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividend receivable from affiliate	(95)
Other	(668)	(802)
Total assets	(19,055)	(19,459)
Liabilities and shareholders’ equity
Unearned premium reserve	(929)	(948)
Loss and loss adjustment expense reserve	(321)	(349)
Long-term debt	(31)	(18)
Intercompany payable	(360)	(360)
Credit derivative liabilities	(32)	(37)
Financial guaranty variable interest entities’ liabilities, at fair value	0	0
Dividend payable to affiliate	(95)
Other	(439)	(532)
Total liabilities	(2,207)	(2,244)
Effect on shareholders' equity (decrease) increase	(16,628)	(16,999)
Noncontrolling interest	(220)	(216)
Total shareholders' equity	(16,848)	(17,215)
Total liabilities and shareholders’ equity	$ (19,055)	$ (19,459)