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Business Developments And Risks And Uncertainties (Narrative) (Detail) shares in Millions, $ in Millions		9 Months Ended
	Oct. 05, 2015 USD ($)	Sep. 30, 2015 USD ($) shares	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Number of operating segments		3
Cutwater sale effective date		Jan. 01, 2015
Mbia Inc [Member]
Business Acquisition [Line Items]
Gain on sale of subsidiary		$ 24
Liquid assets		306	$ 498
Amount released from tax escrow account		$ 228
Treasury stock shares acquired | shares		7.6
National [Member]
Business Acquisition [Line Items]
Impairment of real estate		$ 4
Treasury stock shares acquired | shares		8.0
National [Member] | SubsequentEvent [Member]
Business Acquisition [Line Items]
Dividends from subsidiaries	$ 114
Warburg Pincus [Member]
Business Acquisition [Line Items]
Common Stock Shares Sold | shares		45.7

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