Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Assets:
Fixed-maturity securities	$ 11,168	$ 10,823
Short-term investments	689	590
Other invested assets	173	162
Credit derivative assets	9	13
Liabilities:
Credit derivative liabilities	359	402
Carrying Amount [Member]
Assets:
Fixed-maturity securities	10,479	10,233
Short-term investments	689	590
Other invested assets	149	146
Credit derivative assets	9	13
FG VIEs’ assets, at fair value	781	876
Other assets	213	205
Liabilities:
Financial guaranty insurance contracts	3,537	3,483
Long-term debt	1,307	1,306
Credit derivative liabilities	359	402
FG VIEs’ liabilities with recourse, at fair value	721	807
FG VIEs’ liabilities without recourse, at fair value	134	151
Other liabilities	151	12
Estimated Fair Value [Member]
Assets:
Fixed-maturity securities	10,479	10,233
Short-term investments	689	590
Other invested assets	151	147
Credit derivative assets	9	13
FG VIEs’ assets, at fair value	781	876
Other assets	213	205
Liabilities:
Financial guaranty insurance contracts	8,363	8,738
Long-term debt	1,606	1,546
Credit derivative liabilities	359	402
FG VIEs’ liabilities with recourse, at fair value	721	807
FG VIEs’ liabilities without recourse, at fair value	134	151
Other liabilities	$ 151	$ 12