Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets:
Fixed-maturity securities	$ 11,136	$ 11,301
Short-term investments	911	627
Other invested assets	102	94
Liabilities:
Credit derivative liabilities	258	271
Carrying Amount [Member]
Assets:
Fixed-maturity securities	10,225	10,674
Short-term investments	911	627
Other invested assets	53	60
FG VIEs’ assets, at fair value	627	700
Other assets	246	220
Liabilities:
Financial guaranty insurance contracts	3,533	3,330
Long-term debt	1,264	1,292
Credit derivative liabilities	258	271
FG VIEs’ liabilities with recourse, at fair value	571	627
FG VIEs’ liabilities without recourse, at fair value	108	130
Other liabilities	83	55
Estimated Fair Value [Member]
Assets:
Fixed-maturity securities	10,225	10,674
Short-term investments	911	627
Other invested assets	54	61
FG VIEs’ assets, at fair value	627	700
Other assets	246	220
Liabilities:
Financial guaranty insurance contracts	6,836	7,104
Long-term debt	1,570	1,627
Credit derivative liabilities	258	271
FG VIEs’ liabilities with recourse, at fair value	571	627
FG VIEs’ liabilities without recourse, at fair value	108	130
Other liabilities	$ 83	$ 55