Fair Value Measurement - Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Assets:
Other invested assets	$ 113	$ 118
Other assets	155	135
Carrying Amount
Assets:
Other invested assets	0	1
Other assets	86	97
Liabilities:
Financial guaranty insurance contracts	(2,461)	(2,714)
Long-term debt	(1,223)	(1,235)
Other liabilities	(55)	(14)
Carrying Amount | Consolidated Investment Vehicles
Assets:
Other assets	1	0
Liabilities:
Due from brokers and counterparties	55	0
Due to brokers and counterparties	(162)	0
Estimated Fair Value
Assets:
Other invested assets	2	2
Other assets	86	97
Liabilities:
Financial guaranty insurance contracts	(4,147)	(4,013)
Long-term debt	(1,548)	(1,573)
Other liabilities	(55)	(14)
Estimated Fair Value | Consolidated Investment Vehicles
Assets:
Other assets	1	0
Liabilities:
Due from brokers and counterparties	55	0
Due to brokers and counterparties	$ (162)	$ 0