Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income for basic and diluted EPS									$ 165	$ 361	$ 163
Basic weighted average shares outstanding	76.9	78.8	80.7	81.0	81.3	82.6	84.0	85.7	79.3	83.4	92.0
Options, warrants and non-vested stock									0.8	1.4	1.3
Diluted weighted average shares outstanding	77.6	79.5	81.5	81.0	82.7	84.0	85.2	85.7	80.1	84.8	93.3
Earnings (loss) per share:
Basic	$ 0.16	$ 2.71	$ 0.33	$ (1.08)	$ 2.70	$ 2.96	$ 0.04	$ (1.25)	$ 2.08	$ 4.32	$ 1.78
Diluted	$ 0.16	$ 2.68	$ 0.32	$ (1.08)	$ 2.65	$ 2.91	$ 0.04	$ (1.25)	$ 2.06	$ 4.25	$ 1.75