Common and Preferred Stock (Narrative) (Detail) - USD ($) $ / shares in Units, $ in Millions	2 Months Ended	12 Months Ended
	Feb. 20, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Authorized repurchase of common stock, value		$ 250
Shares repurchased under share repurchase program, average price per share	$ 9.18	$ 9.12	$ 8.21	$ 7.55
Preferred stock shares issued		0	0
Preferred stock shares outstanding		0	0
Preferred Stock Par Or Stated Value Per Share		$ 1	$ 1
Remaining amount available under repurchase program	$ 74	$ 101	$ 202	$ 250
Treasury stock shares acquired	3,000,000.0	11,100,000	5,800,000	43,000,000.0
MBIA Insurance Corporation [Member]
Class of Stock [Line Items]
Preferred Stock Par Or Stated Value Per Share		$ 1,000
Preferred stock liquidation preference		100,000
Preferred Stock [Member] | MBIA Insurance Corporation [Member]
Class of Stock [Line Items]
Shares repurchased under share repurchase program, average price per share		$ 10,900
Preferred stock shares issued		2,759
Preferred stock repurchased during period shares		1,444
Preferred stock, carrying value issued and outstanding		$ 28
Face value of repurchased preferred stock		10.90%
Dividend rate on preferred stock determined rate		LIBOR plus 200 basis points
Unaffiliated Investors [Member] | MBIA Insurance Corporation [Member]
Class of Stock [Line Items]
Preferred stock shares outstanding		1,315