Subsidiary Information - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Total investment portfolio and cash	$ 10,900	$ 10,977
Investment in subsidiaries	0	0
Premiums receivable, net of commissions payable	897	904
Deferred acquisition costs	104	105
Deferred tax asset, net		68
Intercompany loan receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	560	569
Dividends receivable from affiliate	0	0
Other	1,090	980
Total assets	13,551	13,603
Liabilities and shareholders’ equity
Unearned premium reserve	3,437	3,512
Loss and loss adjustment expense reserve	1,032	1,177
Long-term debt	1,232	1,233
Intercompany loan payable	0	0
Credit derivative liabilities	229	209
Deferred tax liabilities, net		0
FG VIEs’ liabilities, at fair value	609	619
Dividends payable to affiliate	0	0
Other	343	298
Total liabilities	6,882	7,048
Effect on shareholders' equity (decrease) increase	6,669	6,555	$ 6,784	$ 6,839
Noncontrolling interest	0	0
Total shareholders' equity	6,669	6,555
Total liabilities and shareholders’ equity	13,551	13,603
AGUS [Member]
Liabilities and shareholders’ equity
Long-term debt	844	844
AGMH [Member]
Liabilities and shareholders’ equity
Long-term debt	470	468
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Assets
Total investment portfolio and cash	43	45
Investment in subsidiaries	6,570	6,440
Premiums receivable, net of commissions payable	0	0
Deferred acquisition costs	0	0
Deferred tax asset, net		0
Intercompany loan receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividends receivable from affiliate	58	60
Other	20	29
Total assets	6,691	6,574
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and loss adjustment expense reserve	0	0
Long-term debt	0	0
Intercompany loan payable	0	0
Credit derivative liabilities	0	0
Deferred tax liabilities, net		0
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	0	0
Other	22	19
Total liabilities	22	19
Effect on shareholders' equity (decrease) increase	6,669	6,555
Noncontrolling interest	0	0
Total shareholders' equity	6,669	6,555
Total liabilities and shareholders’ equity	6,691	6,574
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGUS [Member]
Assets
Total investment portfolio and cash	362	334
Investment in subsidiaries	5,923	5,835
Premiums receivable, net of commissions payable	0	0
Deferred acquisition costs	0	0
Deferred tax asset, net		0
Intercompany loan receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividends receivable from affiliate	0	0
Other	91	66
Total assets	6,376	6,235
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and loss adjustment expense reserve	0	0
Long-term debt	844	844
Intercompany loan payable	50	50
Credit derivative liabilities	0	0
Deferred tax liabilities, net		49
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	58	60
Other	83	3
Total liabilities	1,035	1,006
Effect on shareholders' equity (decrease) increase	5,341	5,229
Noncontrolling interest	0	0
Total shareholders' equity	5,341	5,229
Total liabilities and shareholders’ equity	6,376	6,235
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGMH [Member]
Assets
Total investment portfolio and cash	44	23
Investment in subsidiaries	4,080	3,991
Premiums receivable, net of commissions payable	0	0
Deferred acquisition costs	0	0
Deferred tax asset, net		0
Intercompany loan receivable	0	0
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividends receivable from affiliate	0	0
Other	27	24
Total assets	4,151	4,038
Liabilities and shareholders’ equity
Unearned premium reserve	0	0
Loss and loss adjustment expense reserve	0	0
Long-term debt	470	468
Intercompany loan payable	0	0
Credit derivative liabilities	0	0
Deferred tax liabilities, net		50
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	0	0
Other	73	17
Total liabilities	543	535
Effect on shareholders' equity (decrease) increase	3,608	3,503
Noncontrolling interest	0	0
Total shareholders' equity	3,608	3,503
Total liabilities and shareholders’ equity	4,151	4,038
Reportable Legal Entities [Member] | Other Entities [Member]
Assets
Total investment portfolio and cash	10,880	11,000
Investment in subsidiaries	231	226
Premiums receivable, net of commissions payable	1,059	1,071
Deferred acquisition costs	141	143
Deferred tax asset, net		162
Intercompany loan receivable	50	50
Financial guaranty variable interest entities’ assets, at fair value	560	569
Dividends receivable from affiliate	0	0
Other	2,587	2,437
Total assets	15,508	15,658
Liabilities and shareholders’ equity
Unearned premium reserve	4,358	4,452
Loss and loss adjustment expense reserve	1,312	1,467
Long-term debt	5	5
Intercompany loan payable	300	300
Credit derivative liabilities	260	236
Deferred tax liabilities, net		0
FG VIEs’ liabilities, at fair value	609	619
Dividends payable to affiliate	0	0
Other	750	763
Total liabilities	7,594	7,842
Effect on shareholders' equity (decrease) increase	7,683	7,590
Noncontrolling interest	231	226
Total shareholders' equity	7,914	7,816
Total liabilities and shareholders’ equity	15,508	15,658
Consolidating Adjustments [Member]
Assets
Total investment portfolio and cash	(429)	(425)
Investment in subsidiaries	(16,804)	(16,492)
Premiums receivable, net of commissions payable	(162)	(167)
Deferred acquisition costs	(37)	(38)
Deferred tax asset, net		(94)
Intercompany loan receivable	(50)	(50)
Financial guaranty variable interest entities’ assets, at fair value	0	0
Dividends receivable from affiliate	(58)	(60)
Other	(1,635)	(1,576)
Total assets	(19,175)	(18,902)
Liabilities and shareholders’ equity
Unearned premium reserve	(921)	(940)
Loss and loss adjustment expense reserve	(280)	(290)
Long-term debt	(87)	(84)
Intercompany loan payable	(350)	(350)
Credit derivative liabilities	(31)	(27)
Deferred tax liabilities, net		(99)
FG VIEs’ liabilities, at fair value	0	0
Dividends payable to affiliate	(58)	(60)
Other	(585)	(504)
Total liabilities	(2,312)	(2,354)
Effect on shareholders' equity (decrease) increase	(16,632)	(16,322)
Noncontrolling interest	(231)	(226)
Total shareholders' equity	(16,863)	(16,548)
Total liabilities and shareholders’ equity	(19,175)	(18,902)
AGMH [Member] | Issuer Subsidiary [Member]
Liabilities and shareholders’ equity
Debt instrument, fair value	$ 129	$ 125