Insurance Company Regulatory Requirements - Dividends and Surplus Notes By Insurance Company Subsidiaries (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statutory Accounting Practices [Line Items]
Dividends paid	$ 18	$ 19
AGC [Member] | AGUS [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	52	28
Repurchase of common stock	200	0
AGM [Member] | AGMH [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	73	79
Assured Guaranty Re [Member] | AGL [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	40	40
Municipal Assurance Corp [Member] | Municipal Assurance Corp Holdings [Member] | Affiliated Entity [Member]
Statutory Accounting Practices [Line Items]
Dividends paid	$ 0	$ 12