Long-Term Debt and Credit Facilities Long-Term Debt and Credit Facilities - Purchased Subordinated Debt (Details) - Consolidation, Eliminations [Member] - AGUS [Member] - Assured Guaranty Municipal Holdings Inc [Member] - Junior Subordinated Debt [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Debt repurchase amount	$ 25	$ 6	$ 72	$ 28
Loss on extinguishment of debt	$ 9	$ 2	$ 26	$ 9