Fair Value Measurement - Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Assets:
Other invested assets (includes $12 and $6 measured at fair value)	$ 121	$ 118
Other assets	177	135
Carrying Amount [Member]
Assets:
Other invested assets (includes $12 and $6 measured at fair value)	1	1
Other assets	93	97
Liabilities:
Financial guaranty insurance contracts	(2,668)	(2,714)
Long-term debt	(1,221)	(1,235)
Other liabilities	(18)	(14)
Estimated Fair Value [Member]
Assets:
Other invested assets (includes $12 and $6 measured at fair value)	2	2
Other assets	93	97
Liabilities:
Financial guaranty insurance contracts	(4,306)	(4,013)
Long-term debt	(1,510)	(1,573)
Other liabilities	$ (18)	$ (14)