Insurance Company Regulatory Requirements - Dividends and Surplus Notes By Insurance Company Subsidiaries (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory Accounting Practices [Line Items]
Dividends paid	$ 70	$ 69	$ 72
AGUS [Member] | Affiliated Entity [Member] | AGC [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	107	79	90
AGMH [Member] | Affiliated Entity [Member] | AGM [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	196	247	215
Repayment of surplus notes	0	0	25
Statutory Accounting Practices, Intercompany Transaction, Redemption of Common Stock	101	300	0
Assured Guaranty LTD [Member] | Affiliated Entity [Member] | Assured Guaranty Re [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	125	100	150
AGM [Member] | Affiliated Entity [Member] | MAC [Member]
Statutory Accounting Practices [Line Items]
Repayment of surplus notes	0	100	0
Municipal Assurance Corp Holdings [Member] | Affiliated Entity [Member] | MAC [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	36	0	0
Repayment of surplus notes	0	300	0
Statutory Accounting Practices, Intercompany Transaction, Redemption of Common Stock	$ 250	$ 0	$ 0