Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Assets:
Fixed-maturity securities	$ 11,495	$ 10,823
Short-term investments	949	590
Other invested assets	96	162
Credit derivative assets	3	13
Liabilities:
Credit derivative liabilities	305	402
Carrying Amount [Member]
Assets:
Fixed-maturity securities	10,546	10,233
Short-term investments	949	590
Other invested assets	62	146
Credit derivative assets	3	13
FG VIEs’ assets, at fair value	707	876
Other assets	291	205
Liabilities:
Financial guaranty insurance contracts	3,402	3,483
Long-term debt	1,292	1,306
Credit derivative liabilities	305	402
FG VIEs’ liabilities with recourse, at fair value	657	807
FG VIEs’ liabilities without recourse, at fair value	111	151
Other liabilities	220	12
Estimated Fair Value [Member]
Assets:
Fixed-maturity securities	10,546	10,233
Short-term investments	949	590
Other invested assets	64	147
Credit derivative assets	3	13
FG VIEs’ assets, at fair value	707	876
Other assets	291	205
Liabilities:
Financial guaranty insurance contracts	8,311	8,738
Long-term debt	1,636	1,546
Credit derivative liabilities	305	402
FG VIEs’ liabilities with recourse, at fair value	657	807
FG VIEs’ liabilities without recourse, at fair value	111	151
Other liabilities	$ 220	$ 12