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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income for basic and diluted EPS	$ 142	$ 189	$ 62	$ (91)	$ 13	$ 213	$ 26	$ (87)	$ 302	$ 165	$ 361
Basic weighted average shares outstanding	73.9	75.2	76.0	75.8	76.9	78.8	80.7	81.0	75.2	79.3	83.4
Options, warrants and non-vested stock									0.5	0.8	1.4
Diluted weighted average shares outstanding	74.4	75.7	76.4	75.8	77.6	79.5	81.5	81.0	75.7	80.1	84.8
Earnings (loss) per share:
Basic	$ 1.92	$ 2.52	$ 0.81	$ (1.20)	$ 0.16	$ 2.71	$ 0.33	$ (1.08)	$ 4.01	$ 2.08	$ 4.32
Diluted	$ 1.90	$ 2.50	$ 0.81	$ (1.20)	$ 0.16	$ 2.68	$ 0.32	$ (1.08)	$ 3.98	$ 2.06	$ 4.25