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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income for basic and diluted EPS									$ 361	$ 163	$ 313
Basic weighted average shares outstanding	81.3	82.6	84.0	85.7	87.4	90.4	93.9	96.3	83.4	92.0	103.4
Options, warrants and non-vested stock									1.4	1.3	1.6
Diluted weighted average shares outstanding	82.7	84.0	85.2	85.7	87.4	91.8	95.1	96.3	84.8	93.3	105.0
Earnings (loss) per share:
Basic	$ 2.70	$ 2.96	$ 0.04	$ (1.25)	$ (0.35)	$ 2.32	$ 0.39	$ (0.53)	$ 4.32	$ 1.78	$ 3.02
Diluted	$ 2.65	$ 2.91	$ 0.04	$ (1.25)	$ (0.35)	$ 2.28	$ 0.38	$ (0.53)	$ 4.25	$ 1.75	$ 2.98