Subsidiary Information - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets
Total investment portfolio and cash	$ 11,107	$ 11,539
Investment in subsidiaries	0	0
Premiums receivable, net of commissions payable	916	915
Ceded unearned premium reserve	61	119
Deferred acquisition costs	103	101
Deferred tax asset, net	71	98
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	596	700
Dividends receivable from affiliate	0
Other	885	961
Total assets	13,739	14,433
Liabilities and shareholders’ equity
Unearned premium reserve	3,538	3,475
Loss and loss adjustment expense reserve	1,147	1,444
Long-term debt	1,249	1,292
Intercompany payable	0	0
Credit derivative liabilities	239	271
Deferred tax liabilities, net	0	0
FG VIEs’ liabilities, at fair value	649	757
Dividends payable to affiliate	0
Other	334	355
Total liabilities	7,156	7,594
Effect on shareholders' equity (decrease) increase	6,583	6,839
Noncontrolling interest	0	0
Total shareholders' equity	6,583	6,839
Total liabilities and shareholders’ equity	13,739	14,433
AGUS [Member]
Liabilities and shareholders’ equity
Long-term debt	844	843
AGMH [Member]
Liabilities and shareholders’ equity
Long-term debt	466	461
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Assets
Total investment portfolio and cash	47	36
Investment in subsidiaries	6,448	6,794
Premiums receivable, net of commissions payable	0	0
Ceded unearned premium reserve	0	0
Deferred acquisition costs	0	0
Deferred tax asset, net	0	0
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividends receivable from affiliate	78
Other	25	26
Total assets	6,598	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
Deferred tax liabilities, net	0	0
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	0
Other	15	17
Total liabilities	15	17
Effect on shareholders' equity (decrease) increase	6,583	6,839
Noncontrolling interest	0	0
Total shareholders' equity	6,583	6,839
Total liabilities and shareholders’ equity	6,598	6,856
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGUS [Member]
Assets
Total investment portfolio and cash	257	319
Investment in subsidiaries	5,929	6,126
Premiums receivable, net of commissions payable	0	0
Ceded unearned premium reserve	0	0
Deferred acquisition costs	0	0
Deferred tax asset, net	0	59
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividends receivable from affiliate	0
Other	109	0
Total assets	6,295	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
Deferred tax liabilities, net	30	0
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	51
Other	31	59
Total liabilities	1,016	962
Effect on shareholders' equity (decrease) increase	5,279	5,542
Noncontrolling interest	0	0
Total shareholders' equity	5,279	5,542
Total liabilities and shareholders’ equity	6,295	6,504
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGMH [Member]
Assets
Total investment portfolio and cash	104	28
Investment in subsidiaries	3,967	4,048
Premiums receivable, net of commissions payable	0	0
Ceded unearned premium reserve	0	0
Deferred acquisition costs	0	0
Deferred tax asset, net	0	0
Intercompany receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividends receivable from affiliate	0
Other	56	40
Total assets	4,127	4,116
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and loss adjustment expense reserve	0	0
Long-term debt	466	461
Intercompany payable	0	0
Credit derivative liabilities	0	0
Deferred tax liabilities, net	51	51
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	0
Other	22	20
Total liabilities	539	532
Effect on shareholders' equity (decrease) increase	3,588	3,584
Noncontrolling interest	0	0
Total shareholders' equity	3,588	3,584
Total liabilities and shareholders’ equity	4,127	4,116
Reportable Legal Entities [Member] | Other Entities [Member]
Assets
Total investment portfolio and cash	11,098	11,484
Investment in subsidiaries	219	216
Premiums receivable, net of commissions payable	1,078	1,074
Ceded unearned premium reserve	959	1,002
Deferred acquisition costs	142	144
Deferred tax asset, net	148	93
Intercompany receivable	60	60
Financial guaranty variable interest entities’ assets, at fair value	596	700
Dividends receivable from affiliate	0
Other	1,380	1,643
Total assets	15,680	16,416
Liabilities and shareholders’ equity
Unearned premium reserve	4,478	4,423
Loss and loss adjustment expense reserve	1,429	1,793
Long-term debt	5	6
Intercompany payable	300	300
Credit derivative liabilities	268	308
Deferred tax liabilities, net	0	0
FG VIEs’ liabilities, at fair value	649	757
Dividends payable to affiliate	27
Other	779	740
Total liabilities	7,935	8,327
Effect on shareholders' equity (decrease) increase	7,526	7,873
Noncontrolling interest	219	216
Total shareholders' equity	7,745	8,089
Total liabilities and shareholders’ equity	15,680	16,416
Consolidating Adjustments [Member]
Assets
Total investment portfolio and cash	(399)	(328)
Investment in subsidiaries	(16,563)	(17,184)
Premiums receivable, net of commissions payable	(162)	(159)
Ceded unearned premium reserve	(898)	(883)
Deferred acquisition costs	(39)	(43)
Deferred tax asset, net	(77)	(54)
Intercompany receivable	(60)	(60)
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividends receivable from affiliate	(78)
Other	(685)	(748)
Total assets	(18,961)	(19,459)
Liabilities and shareholders’ equity
Unearned premium reserve	(940)	(948)
Loss and loss adjustment expense reserve	(282)	(349)
Long-term debt	(66)	(18)
Intercompany payable	(360)	(360)
Credit derivative liabilities	(29)	(37)
Deferred tax liabilities, net	(81)	(51)
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	(78)
Other	(513)	(481)
Total liabilities	(2,349)	(2,244)
Effect on shareholders' equity (decrease) increase	(16,393)	(16,999)
Noncontrolling interest	(219)	(216)
Total shareholders' equity	(16,612)	(17,215)
Total liabilities and shareholders’ equity	$ (18,961)	$ (19,459)