Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets:
Fixed-maturity securities	$ 11,301	$ 10,823
Short-term investments	627	590
Other invested assets	94	162
Credit derivative assets	2	13
Liabilities:
Credit derivative liabilities	271	402
Carrying Amount [Member]
Assets:
Fixed-maturity securities	10,674	10,233
Short-term investments	627	590
Other invested assets	60	146
Credit derivative assets	2	13
FG VIEs’ assets, at fair value	700	876
Other assets	218	205
Liabilities:
Financial guaranty insurance contracts	3,330	3,483
Long-term debt	1,292	1,306
Credit derivative liabilities	271	402
FG VIEs’ liabilities with recourse, at fair value	627	807
FG VIEs’ liabilities without recourse, at fair value	130	151
Other liabilities	55	12
Estimated Fair Value [Member]
Assets:
Fixed-maturity securities	10,674	10,233
Short-term investments	627	590
Other invested assets	61	147
Credit derivative assets	2	13
FG VIEs’ assets, at fair value	700	876
Other assets	218	205
Liabilities:
Financial guaranty insurance contracts	7,104	8,738
Long-term debt	1,627	1,546
Credit derivative liabilities	271	402
FG VIEs’ liabilities with recourse, at fair value	627	807
FG VIEs’ liabilities without recourse, at fair value	130	151
Other liabilities	$ 55	$ 12