Fair Value Measurement - Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Assets:
Other invested assets	$ 57	$ 55
Carrying Amount [Member]
Assets:
Other invested assets	1	1
Other assets	91	130
Liabilities:
Financial guaranty insurance contracts	(2,731)	(3,240)
Long-term debt	(1,234)	(1,233)
Other liabilities	(45)	(12)
Estimated Fair Value [Member]
Assets:
Other invested assets	2	2
Other assets	91	130
Liabilities:
Financial guaranty insurance contracts	(5,200)	(5,932)
Long-term debt	(1,549)	(1,496)
Other liabilities	$ (45)	$ (12)