Insurance Company Regulatory Requirements - Dividends and Surplus Notes By Insurance Company Subsidiaries (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statutory Accounting Practices [Line Items]
Dividends paid			$ 37	$ 36
AGC [Member] | AGUS [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	$ 24	$ 23	76	51
Repurchase of common stock	0	0	200	0
AGM [Member] | AGMH [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	0	0	73	79
Assured Guaranty Re [Member] | AGL [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	40	40	80	80
Municipal Assurance Corp [Member] | Municipal Assurance Corp Holdings [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	$ 15	$ 12	$ 15	$ 24