Subsidiary Information - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Total investment portfolio and cash	$ 10,977	$ 11,539
Investment in subsidiaries	0	0
Premiums receivable, net of commissions payable	904	915
Ceded unearned premium reserve	59	119
Deferred acquisition costs	105	101	$ 106	$ 114
Deferred tax asset, net	68	98
Intercompany loan receivable	0	0
FG VIEs’ assets, at fair value	569	700
Dividends receivable from affiliate	0
Other	921	961
Total assets	13,603	14,433
Liabilities and shareholders’ equity
Unearned premium reserve	3,512	3,475
Loss and LAE reserve	1,177	1,444
Long-term debt	1,233	1,292
Intercompany loan payable	0	0
Credit derivative liabilities	209	271
Deferred tax liabilities, net	0	0
FG VIEs’ liabilities, at fair value	619	757
Dividends payable to affiliate	0
Other	298	355
Total liabilities	7,048	7,594
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	6,555	6,839	$ 6,504	$ 6,063
Noncontrolling interest	0	0
Total shareholders' equity	6,555	6,839
Total liabilities and shareholders’ equity	13,603	14,433
AGUS (Issuer) [Member] | AGMH [Member]
Liabilities and shareholders’ equity
Debt Instrument, Fair Value Disclosure	125	28
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Assets
Total investment portfolio and cash	45	36
Investment in subsidiaries	6,440	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 loan receivable	0	0
FG VIEs’ assets, at fair value	0	0
Dividends receivable from affiliate	60
Other	29	26
Total assets	6,574	6,856
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and LAE reserve	0	0
Long-term debt	0	0
Intercompany loan 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	19	17
Total liabilities	19	17
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	6,555	6,839
Noncontrolling interest	0	0
Total shareholders' equity	6,555	6,839
Total liabilities and shareholders’ equity	6,574	6,856
Reportable Legal Entities [Member] | AGUS (Issuer) [Member]
Assets
Total investment portfolio and cash	334	319
Investment in subsidiaries	5,835	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 loan receivable	0	0
FG VIEs’ assets, at fair value	0	0
Dividends receivable from affiliate	0
Other	66	0
Total assets	6,235	6,504
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and LAE reserve	0	0
Long-term debt	844	843
Intercompany loan payable	50	60
Credit derivative liabilities	0	0
Deferred tax liabilities, net	49	0
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	60
Other	3	59
Total liabilities	1,006	962
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	5,229	5,542
Noncontrolling interest	0	0
Total shareholders' equity	5,229	5,542
Total liabilities and shareholders’ equity	6,235	6,504
Reportable Legal Entities [Member] | AGMH (Issuer) [Member]
Assets
Total investment portfolio and cash	23	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
Deferred tax asset, net	0	0
Intercompany loan receivable	0	0
FG VIEs’ assets, at fair value	0	0
Dividends receivable from affiliate	0
Other	24	40
Total assets	4,038	4,116
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and LAE reserve	0	0
Long-term debt	468	461
Intercompany loan payable	0	0
Credit derivative liabilities	0	0
Deferred tax liabilities, net	50	51
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	0
Other	17	20
Total liabilities	535	532
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	3,503	3,584
Noncontrolling interest	0	0
Total shareholders' equity	3,503	3,584
Total liabilities and shareholders’ equity	4,038	4,116
Reportable Legal Entities [Member] | Other Entities [Member]
Assets
Total investment portfolio and cash	11,000	11,484
Investment in subsidiaries	226	216
Premiums receivable, net of commissions payable	1,071	1,074
Ceded unearned premium reserve	958	1,002
Deferred acquisition costs	143	144
Deferred tax asset, net	162	93
Intercompany loan receivable	50	60
FG VIEs’ assets, at fair value	569	700
Dividends receivable from affiliate	0
Other	1,479	1,643
Total assets	15,658	16,416
Liabilities and shareholders’ equity
Unearned premium reserve	4,452	4,423
Loss and LAE reserve	1,467	1,793
Long-term debt	5	6
Intercompany loan payable	300	300
Credit derivative liabilities	236	308
Deferred tax liabilities, net	0	0
FG VIEs’ liabilities, at fair value	619	757
Dividends payable to affiliate	0
Other	763	740
Total liabilities	7,842	8,327
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	7,590	7,873
Noncontrolling interest	226	216
Total shareholders' equity	7,816	8,089
Total liabilities and shareholders’ equity	15,658	16,416
Consolidating Adjustments [Member]
Assets
Total investment portfolio and cash	(425)	(328)
Investment in subsidiaries	(16,492)	(17,184)
Premiums receivable, net of commissions payable	(167)	(159)
Ceded unearned premium reserve	(899)	(883)
Deferred acquisition costs	(38)	(43)
Deferred tax asset, net	(94)	(54)
Intercompany loan receivable	(50)	(60)
FG VIEs’ assets, at fair value	0	0
Dividends receivable from affiliate	(60)
Other	(677)	(748)
Total assets	(18,902)	(19,459)
Liabilities and shareholders’ equity
Unearned premium reserve	(940)	(948)
Loss and LAE reserve	(290)	(349)
Long-term debt	(84)	(18)
Intercompany loan payable	(350)	(360)
Credit derivative liabilities	(27)	(37)
Deferred tax liabilities, net	(99)	(51)
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	(60)
Other	(504)	(481)
Total liabilities	(2,354)	(2,244)
TOTAL SHAREHOLDERS’ EQUITY ATTRIBUTABLE TO ASSURED GUARANTY LTD.	(16,322)	(16,999)
Noncontrolling interest	(226)	(216)
Total shareholders' equity	(16,548)	(17,215)
Total liabilities and shareholders’ equity	$ (18,902)	$ (19,459)