Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets:
Fixed-maturity securities	$ 11,048	$ 11,301
Short-term investments	751	627
Other invested assets	103	94
Liabilities:
Credit derivative liabilities	237	271
Carrying Amount [Member]
Assets:
Fixed-maturity securities	10,297	10,674
Short-term investments	751	627
Other invested assets	55	60
FG VIEs’ assets, at fair value	651	700
Other assets	226	220
Liabilities:
Financial guaranty insurance contracts	3,217	3,330
Long-term debt	1,281	1,292
Credit derivative liabilities	237	271
FG VIEs’ liabilities with recourse, at fair value	598	627
FG VIEs’ liabilities without recourse, at fair value	110	130
Other liabilities	58	55
Estimated Fair Value [Member]
Assets:
Fixed-maturity securities	10,297	10,674
Short-term investments	751	627
Other invested assets	57	61
FG VIEs’ assets, at fair value	651	700
Other assets	226	220
Liabilities:
Financial guaranty insurance contracts	6,880	7,104
Long-term debt	1,618	1,627
Credit derivative liabilities	237	271
FG VIEs’ liabilities with recourse, at fair value	598	627
FG VIEs’ liabilities without recourse, at fair value	110	130
Other liabilities	$ 58	$ 55