Fair Value Measurement - Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets:
Other invested assets	$ 260	$ 214
Other assets	127	145
Liabilities:
Repurchase agreement liability	19
Carrying Amount
Assets:
Other invested assets	0	1
Other assets	91	83
Liabilities:
Financial guaranty insurance contracts	(2,163)	(2,464)
Long-term debt	(1,671)	(1,224)
Other liabilities	(40)	(27)
Carrying Amount | Consolidated Investment Vehicles
Liabilities:
Due from brokers and counterparties	116	52
Due to brokers and counterparties	(642)	(290)
Estimated Fair Value
Assets:
Other invested assets	2	3
Other assets	91	83
Liabilities:
Financial guaranty insurance contracts	(2,056)	(3,882)
Long-term debt	(1,845)	(1,561)
Other liabilities	(40)	(27)
Estimated Fair Value | Consolidated Investment Vehicles
Liabilities:
Due from brokers and counterparties	116	52
Due to brokers and counterparties	$ (642)	$ (290)