Common and Preferred Stock (Schedule of Share Repurchases) (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Share Repurchases [Abstract]
Number of shares repurchased	43,000,000	16,600,000	39,900,000
Average price paid per share	$ 7.55	$ 6.37	$ 7.6
Remaining authorization as of December 31	$ 250	$ 88	$ 94