Business Developments And Risks And Uncertainties (Narrative) (Detail) $ / shares in Units, $ in Millions		12 Months Ended
	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($) segments $ / shares shares	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Number of operating segments | segments		3
Warrants exercised | shares		11,850,000
Warrant exercise price | $ / shares		$ 9.59
Warrants outstanding | shares		0
Treasury shares issued (in shares) | shares		1,300,000
Zohar [Member]
Business Acquisition [Line Items]
Aggregate claim amount		$ 919
Puerto Rico [Member]
Business Acquisition [Line Items]
Claims payments		$ 276
SubsequentEvent [Member] | Puerto Rico [Member]
Business Acquisition [Line Items]
Claims payments	$ 65
Assured Guaranty Corp [Member]
Business Acquisition [Line Items]
Payment for sale of subsidiary			$ 23
Assured Guaranty Corp [Member] | Zohar II [Member]
Business Acquisition [Line Items]
Outstanding bonds			$ 347
MBIA UK [Member]
Business Acquisition [Line Items]
Sale effective date			Jan. 10, 2017
Gain (loss) on adjusting MBIA UK to its fair value less costs to sell			$ 5