Business Developments And Risks And Uncertainties (Narrative) (Detail) $ in Millions		9 Months Ended
	Oct. 06, 2016 USD ($)	Sep. 30, 2016 USD ($) segments	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Number of operating segments | segments		3
Cutwater sale effective date		Jan. 01, 2015
Mbia Inc [Member]
Business Acquisition [Line Items]
Liquid assets		$ 237	$ 416
Amount released from tax escrow account		105
Mbia Corp [Member] | Zohar [Member]
Business Acquisition [Line Items]
Claims payments		149
Insurance par exposure		770
National [Member]
Business Acquisition [Line Items]
Amount paid into tax escrow account		73
National [Member] | Puerto Rico [Member]
Business Acquisition [Line Items]
Claims payments		173
National [Member] | SubsequentEvent [Member]
Business Acquisition [Line Items]
Dividends from subsidiaries	$ 118
Assured Guaranty Corp [Member]
Business Acquisition [Line Items]
Payment for sale of subsidiary		23
Assured Guaranty Corp [Member] | Zohar [Member]
Business Acquisition [Line Items]
Insurance par exposure		$ 347