Business Developments And Risks And Uncertainties (Narrative) (Detail) $ / shares in Units, $ in Millions		12 Months Ended
	Jan. 19, 2016 USD ($)	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 $ / shares	Feb. 29, 2016 USD ($)
Business Acquisition [Line Items]
Number of operating segments		3
Cutwater sale effective date		Jan. 01, 2015
Treasury stock shares acquired | shares		39,900,000	3,300,000
Shares repurchased under share repurchase program, average price per share | $ / shares		$ 7.6	$ 10.45	$ 0
PREPA [Member]
Business Acquisition [Line Items]
Outatanding bonds		$ 8,100
Mbia Inc [Member]
Business Acquisition [Line Items]
Gain on sale of subsidiary		24
Liquid assets		416	$ 498
Amount released from tax escrow account		$ 228
Treasury stock shares acquired | shares		7,600,000
Mbia Inc [Member] | SubsequentEvent [Member]
Business Acquisition [Line Items]
Amount released from tax escrow account	$ 105
Mbia Corp [Member] | SubsequentEvent [Member] | Zohar [Member]
Business Acquisition [Line Items]
Insurance par exposure					$ 776
National [Member]
Business Acquisition [Line Items]
Impairment of real estate		$ 4
Dividends from subsidiaries		$ 114	$ 220
Treasury stock shares acquired | shares		8,000,000
National [Member] | PREPA [Member]
Business Acquisition [Line Items]
Insurance par exposure		$ 1,400
Warburg Pincus [Member]
Business Acquisition [Line Items]
Common Stock Shares Sold | shares		45,700,000