Business Developments And Risks And Uncertainties (Narrative) (Detail) $ / shares in Units, $ in Millions					12 Months Ended
		Jan. 20, 2017 USD ($)	Jan. 10, 2017 USD ($)	Jan. 01, 2017 USD ($)	Dec. 31, 2016 USD ($) segments $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares
Business Acquisition [Line Items]
Number of operating segments | segments					3
Fixed-to-Floating rate Surplus Notes					14.00%
Dividends from subsidiaries					$ 118	$ 114
Income (loss) before income taxes					$ (339)	$ 289	$ 641
Treasury stock shares acquired | shares					16,600,000	39,900,000	3,300,000
Shares repurchased under share repurchase program, average price per share | $ / shares					$ 6.37	$ 7.6	$ 10.45
Included in AOCI	[1]				$ 270
Accumulated Other Comprehensive Income (Loss) [Member]
Business Acquisition [Line Items]
Included in AOCI					97
Zohar I [Member]
Business Acquisition [Line Items]
Claims payments						$ 149
SubsequentEvent [Member]
Business Acquisition [Line Items]
Proceeds from Senior Lenders			$ 325
SubsequentEvent [Member] | Zohar II [Member]
Business Acquisition [Line Items]
Claims payments		$ 770
SubsequentEvent [Member] | Puerto Rico [Member]
Business Acquisition [Line Items]
Claims payments				$ 24
Mbia Inc [Member]
Business Acquisition [Line Items]
Liquid assets					403
Mbia Inc [Member] | SubsequentEvent [Member]
Business Acquisition [Line Items]
Proceeds from subordinated financing			38
Mbia Corp [Member] | SubsequentEvent [Member] | Zohar II [Member]
Business Acquisition [Line Items]
Claims payments		$ 60
National [Member] | Puerto Rico [Member]
Business Acquisition [Line Items]
Claims payments					$ 173
Cutwater Holdings [Member]
Business Acquisition [Line Items]
Sale effective date					Jan. 01, 2015
Assured Guaranty Corp [Member] | Zohar II [Member]
Business Acquisition [Line Items]
Outstanding bonds					$ 347
Assured Guaranty Corp [Member] | SubsequentEvent [Member]
Business Acquisition [Line Items]
Payment for sale of subsidiary			$ 23
MBIA UK [Member]
Business Acquisition [Line Items]
Loss recorded on adjustment of carrying value to fair value less costs to sell					114
Insurance par exposure					18,600
Income (loss) before income taxes					$ 45	$ (5)	$ 71
MBIA UK [Member] | SubsequentEvent [Member]
Business Acquisition [Line Items]
Sale effective date			Jan. 10, 2017

[1] Includes $97 million of losses related to items that continue to be included in accumulated other comprehensive income (loss) until ultimate sale.