Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets (liabilities):
Other invested assets (includes $6 and $7 measured at fair value)	$ 118	$ 55
Other assets	3	139
Carrying Amount [Member]
Assets (liabilities):
Other invested assets (includes $6 and $7 measured at fair value)	1	1
Other assets	97	130
Liabilities:
Financial guaranty insurance contracts	(2,714)	(3,240)
Long-term debt	(1,235)	(1,233)
Other liabilities	(14)	(12)
Estimated Fair Value [Member]
Assets (liabilities):
Other invested assets (includes $6 and $7 measured at fair value)	2	2
Other assets	97	130
Liabilities:
Financial guaranty insurance contracts	(4,013)	(5,932)
Long-term debt	(1,573)	(1,496)
Other liabilities	$ (14)	$ (12)