Subsidiary Information - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets
Total investment portfolio and cash	$ 11,423	$ 11,539
Investment in subsidiaries	0	0
Premiums receivable, net of commissions payable	932	915
Ceded unearned premium reserve	66	119
Deferred acquisition costs	102	101
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	627	700
Dividend receivable from affiliate	0
Other	982	1,059
Total assets	14,132	14,433
Liabilities and shareholders’ equity
Unearned premium reserve	3,635	3,475
Loss and loss adjustment expense reserve	1,327	1,444
Long-term debt	1,264	1,292
Intercompany payable	0	0
Credit derivative liabilities	258	271
FG VIEs’ liabilities, at fair value	679	757
Dividend payable to affiliate	0
Other	335	355
Total liabilities	7,498	7,594
Effect on shareholders' equity (decrease) increase	6,634	6,839
Noncontrolling interest	0	0
Total shareholders' equity	6,634	6,839
Total liabilities and shareholders’ equity	14,132	14,433
AGUS [Member]
Liabilities and shareholders’ equity
Long-term debt	844	843
AGMH [Member]
Liabilities and shareholders’ equity
Long-term debt	464	461
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Assets
Total investment portfolio and cash	33	36
Investment in subsidiaries	6,538	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	51
Other	24	26
Total assets	6,646	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
FG VIEs’ liabilities, at fair value	0	0
Dividend payable to affiliate	0
Other	12	17
Total liabilities	12	17
Effect on shareholders' equity (decrease) increase	6,634	6,839
Noncontrolling interest	0	0
Total shareholders' equity	6,634	6,839
Total liabilities and shareholders’ equity	6,646	6,856
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGUS [Member]
Assets
Total investment portfolio and cash	386	319
Investment in subsidiaries	5,893	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	58	59
Total assets	6,337	6,504
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and loss adjustment expense reserve	0	0
Long-term debt	844	843
Intercompany payable	60	60
Credit derivative liabilities	0	0
FG VIEs’ liabilities, at fair value	0	0
Dividend payable to affiliate	51
Other	39	59
Total liabilities	994	962
Effect on shareholders' equity (decrease) increase	5,343	5,542
Noncontrolling interest	0	0
Total shareholders' equity	5,343	5,542
Total liabilities and shareholders’ equity	6,337	6,504
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGMH [Member]
Assets
Total investment portfolio and cash	21	28
Investment in subsidiaries	3,991	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	50	40
Total assets	4,062	4,116
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and loss adjustment expense reserve	0	0
Long-term debt	464	461
Intercompany payable	0	0
Credit derivative liabilities	0	0
FG VIEs’ liabilities, at fair value	0	0
Dividend payable to affiliate	0
Other	70	71
Total liabilities	534	532
Effect on shareholders' equity (decrease) increase	3,528	3,584
Noncontrolling interest	0	0
Total shareholders' equity	3,528	3,584
Total liabilities and shareholders’ equity	4,062	4,116
Reportable Legal Entities [Member] | Other Entities [Member]
Assets
Total investment portfolio and cash	11,358	11,484
Investment in subsidiaries	220	216
Premiums receivable, net of commissions payable	1,127	1,074
Ceded unearned premium reserve	986	1,002
Deferred acquisition costs	143	144
Intercompany receivable	60	60
Financial guaranty variable interest entities’ assets, at fair value	627	700
Dividend receivable from affiliate	0
Other	1,554	1,736
Total assets	16,075	16,416
Liabilities and shareholders’ equity
Unearned premium reserve	4,610	4,423
Loss and loss adjustment expense reserve	1,648	1,793
Long-term debt	5	6
Intercompany payable	300	300
Credit derivative liabilities	288	308
FG VIEs’ liabilities, at fair value	679	757
Dividend payable to affiliate	0
Other	731	740
Total liabilities	8,261	8,327
Effect on shareholders' equity (decrease) increase	7,594	7,873
Noncontrolling interest	220	216
Total shareholders' equity	7,814	8,089
Total liabilities and shareholders’ equity	16,075	16,416
Consolidation, Eliminations [Member]
Assets
Total investment portfolio and cash	(375)	(328)
Investment in subsidiaries	(16,642)	(17,184)
Premiums receivable, net of commissions payable	(195)	(159)
Ceded unearned premium reserve	(920)	(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	(51)
Other	(704)	(802)
Total assets	(18,988)	(19,459)
Liabilities and shareholders’ equity
Unearned premium reserve	(975)	(948)
Loss and loss adjustment expense reserve	(321)	(349)
Long-term debt	(49)	(18)
Intercompany payable	(360)	(360)
Credit derivative liabilities	(30)	(37)
FG VIEs’ liabilities, at fair value	0	0
Dividend payable to affiliate	(51)
Other	(517)	(532)
Total liabilities	(2,303)	(2,244)
Effect on shareholders' equity (decrease) increase	(16,465)	(16,999)
Noncontrolling interest	(220)	(216)
Total shareholders' equity	(16,685)	(17,215)
Total liabilities and shareholders’ equity	$ (18,988)	$ (19,459)