Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Summary of Percentage Ceded to Reinsurers and Reinsurance Recoverable and Rating Levels (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Insurance [Abstract]
Loss Reserves Ceded To Reinsurers	$ 36	$ 26	$ 26	$ 23
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 1	$ 0	$ 0