Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets:
Other invested assets	$ 55	$ 94
Carrying Amount [Member]
Assets:
Other invested assets	1	8
Other assets (2)	130	97
Liabilities:
Financial guaranty insurance contracts	3,240	3,330
Long-term debt	1,233	1,292
Other liabilities (2)	12	55
Estimated Fair Value [Member]
Assets:
Other invested assets	2	9
Other assets (2)	130	97
Liabilities:
Financial guaranty insurance contracts	5,932	7,104
Long-term debt	1,496	1,627
Other liabilities (2)	$ 12	$ 55