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
	Mar. 31, 2019	Mar. 31, 2018
Debt Instrument [Line Items]
Debt repurchase amount	$ 3	$ 20
Loss on extinguishment of debt	$ (1)	$ (7)