Fair Value Measurement - Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets:
Other invested assets	$ 61	$ 55
Carrying Amount [Member]
Assets:
Other invested assets	1	1
Other assets	119	130
Liabilities:
Financial guaranty insurance contracts	2,987	3,240
Long-term debt	1,232	1,233
Other liabilities	48	12
Estimated Fair Value [Member]
Assets:
Other invested assets	2	2
Other assets	119	130
Liabilities:
Financial guaranty insurance contracts	5,740	5,932
Long-term debt	1,517	1,496
Other liabilities	$ 48	$ 12