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			6 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income (Numerator):
Net Income	$ 53	$ 53	$ 53	$ 106	$ 128
Weighted average shares outstanding for basic computation:
Average shares outstanding	253		251	253	251
Adjustment to shares outstanding	0		0	0	0
Weighted Average Shares Outstanding for Computation of Basic Earnings Per Share of Common Stock	253		251	253	251
Net effect of potentially dilutive shares	1		1	1
Weighted Average Shares Outstanding for Computation of Diluted Earnings Per Share of Common Stock	254		252	254	251
Basic and Diluted Earnings per Share
Basic and Diluted earnings per share	$ 0.21		$ 0.21	$ 0.42	$ 0.51