Insurance Company Regulatory Requirements - Dividends and Surplus Notes By Insurance Company Subsidiaries (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statutory Accounting Practices [Line Items]
Dividends paid			$ 55	$ 53
AGC [Member] | AGUS [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	$ 15	$ 15	91	66
Repurchase of common stock	0	0	200	0
AGM [Member] | AGMH [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	58	63	131	142
Assured Guaranty Re [Member] | AGL [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	18	45	98	125
Municipal Assurance Corp [Member] | Municipal Assurance Corp Holdings [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	12	12	27	36
Repurchase of common stock	$ 0	$ 250	$ 0	$ 250