Financial Guarantee Insurance Contracts - Summary of Gross Premium Receivable Roll-Forward (Direct and Assumed Contracts) (Details) $ in Millions	Dec. 31, 2020 USD ($)
Premiums Receivable, Gross	$ 387
Public Finance [Member]
Premiums Receivable, Gross	185
Housing Revenue [Member] | Public Finance [Member]
Premiums Receivable, Gross	168
Surveillance Category One [Member]
Premiums Receivable, Gross	302
Surveillance Category One [Member] | Public Finance [Member]
Premiums Receivable, Gross	157
Surveillance Category One [Member] | Housing Revenue [Member] | Public Finance [Member]
Premiums Receivable, Gross	$ 155