Schedule I - Condensed Financial Information of Parent Company (Statements of Income (Loss)) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Income Statements, Captions [Line Items]
Operating Revenue	$ 1,118	$ 1,313	$ 1,117	$ 1,330	$ 1,091	$ 1,344	$ 1,051	$ 1,180	$ 4,878	$ 4,666	$ 4,625
Operating Expenses
Other operation and maintenance									924	851	898
Total operating expenses	1,004	1,147	966	1,157	936	1,109	906	1,047	4,274	3,998	4,084
Operating Income (Loss)	114	166	151	173	155	235	145	133	604	668	541
Other Income (Expenses)
Interest expense									(268)	(273)	(256)
Income from equity investments										2	1
Total Other Income (Expenses)	(66)	(53)	(53)	(52)	(58)	(60)	(62)	(59)	(224)	(239)	(220)
Income Tax (Benefit) Expense Related to Continuing Operations	13	34	45	46	39	65	30	185	138	319	103
Net Income from Continuing Operations					58	110	53	(111)	242	110	218
(Loss) Income from Discontinued Operations, net of Income Taxes						8	(11)	(319)		(322)	67
Net Income (Loss)	35	79	53	75	58	118	42	(430)	242	(212)	285
Comprehensive Income (Loss)									230	(198)	300
Earnings Per Share
Basic earnings per share of common stock from continuing operations									$ 0.96	$ 0.45	$ 0.95
Basic (loss) earnings per share of common stock from discontinued operations										$ (1.31)	$ 0.30
Basic earnings (loss) per share of common stock									$ 0.96	$ (0.86)	$ 1.25
Diluted earnings per share of common stock from continuing operations									$ 0.96	$ 0.45	$ 0.95
Diluted (loss) earnings per share of common stock from discontinued operations										$ (1.31)	$ 0.29
Diluted earnings (loss) per share of common stock									$ 0.96	$ (0.86)	$ 1.24
Parent Company [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenue									0	0	0
Operating Expenses
Other operation and maintenance									31	1	1
Total operating expenses									31	1	1
Operating Income (Loss)									(31)	(1)	(1)
Other Income (Expenses)
Interest expense									(43)	(42)	(33)
Income from equity investments									291	204	237
Total Other Income (Expenses)									248	162	204
Income from Continuing Operations Before Income Tax									217	161	203
Income Tax (Benefit) Expense Related to Continuing Operations									(25)	51	(15)
Net Income from Continuing Operations									242	110	218
(Loss) Income from Discontinued Operations, net of Income Taxes										(322)	67
Net Income (Loss)									242	(212)	285
Comprehensive Income (Loss)									$ 230	$ (198)	$ 300
Earnings Per Share
Basic earnings per share of common stock from continuing operations									$ 0.96	$ 0.45	$ 0.95
Basic (loss) earnings per share of common stock from discontinued operations										$ (1.31)	$ 0.30
Basic earnings (loss) per share of common stock									$ 0.96	$ (0.86)	$ 1.25
Diluted earnings per share of common stock from continuing operations									$ 0.96	$ 0.45	$ 0.95
Diluted (loss) earnings per share of common stock from discontinued operations										$ (1.31)	$ 0.29
Diluted earnings (loss) per share of common stock									$ 0.96	$ (0.86)	$ 1.24