Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Assets:
Fixed-maturity securities	$ 11,183	$ 10,823
Short-term investments	678	590
Other invested assets	88	162
Credit derivative assets	6	13
Liabilities:
Credit derivative liabilities	367	402
Carrying Amount [Member]
Assets:
Fixed-maturity securities	10,505	10,233
Short-term investments	678	590
Other invested assets	65	146
Credit derivative assets	6	13
FG VIEs’ assets, at fair value	757	876
Other assets	245	205
Liabilities:
Financial guaranty insurance contracts	3,498	3,483
Long-term debt	1,294	1,306
Credit derivative liabilities	367	402
FG VIEs’ liabilities with recourse, at fair value	689	807
FG VIEs’ liabilities without recourse, at fair value	131	151
Other liabilities	62	12
Estimated Fair Value [Member]
Assets:
Fixed-maturity securities	10,505	10,233
Short-term investments	678	590
Other invested assets	66	147
Credit derivative assets	6	13
FG VIEs’ assets, at fair value	757	876
Other assets	245	205
Liabilities:
Financial guaranty insurance contracts	8,228	8,738
Long-term debt	1,619	1,546
Credit derivative liabilities	367	402
FG VIEs’ liabilities with recourse, at fair value	689	807
FG VIEs’ liabilities without recourse, at fair value	131	151
Other liabilities	$ 62	$ 12