Fair Value Measurement - Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Assets:
Other invested assets (includes $15 and $6 measured at fair value)	$ 122	$ 118
Other assets	160	135
Carrying Amount [Member]
Assets:
Other invested assets (includes $15 and $6 measured at fair value)	0	1
Other assets	84	97
Liabilities:
Financial guaranty insurance contracts	(2,724)	(2,714)
Long-term debt	(1,222)	(1,235)
Other liabilities	(36)	(14)
Carrying Amount [Member] | Consolidated Investment Vehicles [Member]
Liabilities:
Due from Correspondent Brokers	41	0
Due to Correspondent Brokers	(400)	0
Estimated Fair Value [Member]
Assets:
Other invested assets (includes $15 and $6 measured at fair value)	2	2
Other assets	84	97
Liabilities:
Financial guaranty insurance contracts	(4,568)	(4,013)
Long-term debt	(1,517)	(1,573)
Other liabilities	(36)	(14)
Estimated Fair Value [Member] | Consolidated Investment Vehicles [Member]
Liabilities:
Due from Correspondent Brokers	41	0
Due to Correspondent Brokers	$ (400)	$ 0