Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets:
Fixed-maturity securities	$ 10,930	$ 11,301
Short-term investments	738	627
Other invested assets	95	94
Liabilities:
Credit derivative liabilities	239	271
Carrying Amount [Member]
Assets:
Fixed-maturity securities	10,192	10,674
Short-term investments	738	627
Other invested assets	49	60
FG VIEs’ assets, at fair value	596	700
Other assets	235	220
Liabilities:
Financial guaranty insurance contracts	3,237	3,330
Long-term debt	1,249	1,292
Credit derivative liabilities	239	271
FG VIEs’ liabilities with recourse, at fair value	545	627
FG VIEs’ liabilities without recourse, at fair value	104	130
Other liabilities	72	55
Estimated Fair Value [Member]
Assets:
Fixed-maturity securities	10,192	10,674
Short-term investments	738	627
Other invested assets	51	61
FG VIEs’ assets, at fair value	596	700
Other assets	235	220
Liabilities:
Financial guaranty insurance contracts	6,254	7,104
Long-term debt	1,520	1,627
Credit derivative liabilities	239	271
FG VIEs’ liabilities with recourse, at fair value	545	627
FG VIEs’ liabilities without recourse, at fair value	104	130
Other liabilities	$ 72	$ 55