Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Summary of Percentage Ceded to Reinsurers and Reinsurance Recoverable and Rating Levels (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Insurance [Abstract]
Loss Reserves Ceded To Reinsurers	$ 33	$ 33	$ 35	$ 26
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 0	$ 1	$ 1