Equity and Earnings Per Share - Calculation of Earnings Per Share of Common Stock (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income (Numerator):
Net Income	$ 91	$ 53	$ 53	$ 79	$ 197	$ 207
Weighted average shares outstanding for basic computation:
Average shares outstanding	254			252	253	251
Adjustment to shares outstanding	0			0	0	0
Weighted Average Shares Outstanding for Computation of Basic Earnings Per Share of Common Stock	254			252	253	251
Net effect of potentially dilutive shares					1	1
Weighted Average Shares Outstanding for Computation of Diluted Earnings Per Share of Common Stock	254			252	254	252
Basic and Diluted Earnings per Share
Basic and Diluted earnings per share	$ 0.36			$ 0.31	$ 0.78	$ 0.82