Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		$ 2,172	$ 2,802
Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		82
Gross unrealized losses		(6)
Loss and loss adjustment expense reserves	[1]	19
Asset-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		183
Gross unrealized losses		(3)
Asset-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	51
Gross unrealized losses	[2]	(2)
Loss and loss adjustment expense reserves	[1],[2]	4
Mortgage-backed [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		609
Gross unrealized losses		(18)
Mortgage-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	17
Gross unrealized losses	[2]	(3)
Loss and loss adjustment expense reserves	[1],[2]	15
Corporate Obligations [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		586
Gross unrealized losses		(102)
All Other Securities [Member] | Wrapped Securities [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		14
Gross unrealized losses		(1)
Loss and loss adjustment expense reserves	[1]	0
All Other Securities [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value	[2]	14
Gross unrealized losses	[2]	(1)
Loss and loss adjustment expense reserves	[1],[2]	0
All Other Securities [Member] | Wrapped Securities [Member] | Other Insurers [Member]
Schedule Of Investments [Line Items]
Total Available-For-Sale, Fair Value		0
Gross unrealized losses		0
Loss and loss adjustment expense reserves	[1]	$ 0

[1] Insurance loss reserve estimates are based on the proportion of par value owned to the total amount of par value insured.

[2] Includes investments insured by MBIA Corp. and National.