Schedule I - Condensed Financial Information of Parent Company (Statements of (Loss) Income) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating Revenue	$ 1,091	$ 1,344	$ 1,051	$ 1,180	$ 1,056	$ 1,389	$ 1,057	$ 1,123	$ 4,666	$ 4,625	$ 4,964
Operating Expenses
Other operation and maintenance									(851)	(898)	(889)
Total operating expenses	(936)	(1,109)	(906)	(1,047)	(954)	(1,188)	(932)	(1,010)	(3,998)	(4,084)	(4,411)
Operating (Loss) Income	155	235	145	133	102	201	125	113	668	541	553
Other Income (Expenses)
Interest expense									(273)	(256)	(242)
Income from equity investments									2	1	(3)
Impairment Losses									(4)	(12)
Total Other Expenses	(58)	(60)	(62)	(59)	(57)	(57)	(52)	(54)	(239)	(220)	(217)
Income from Continuing Operations Before Income Tax	97	175	83	74	45	144	73	59	429	321	336
Income Tax Expense (Benefit) Related to Continuing Operations	39	65	30	185	11	57	26	9	319	103	114
Net Income from Continuing Operations	58	110	53	(111)	34	87	47	50	110	218	222
(Loss)Income from Discontinued Operations, net of Income Taxes		8	(11)	(319)	9	25	15	18	(322)	67	35
Net (Loss) Income	58	118	42	(430)	43	112	62	68	(212)	285	257
Comprehensive (Loss) Income									(198)	300	300
Earnings Per Share
Basic earnings per share of common stock from continuing operations					$ 0.15	$ 0.38	$ 0.20	$ 0.22	$ 0.45	$ 0.95	$ 0.98
Basic (loss) earnings per share of common stock from Discontinued Operations					$ 0.03	$ 0.11	$ 0.07	$ 0.08	$ (1.31)	$ 0.30	$ 0.16
Basic (loss) earnings per share of common stock					$ 0.18	$ 0.49	$ 0.27	$ 0.30	$ (0.86)	$ 1.25	$ 1.14
Diluted earnings per share of common stock from continuing operations					$ 0.15	$ 0.38	$ 0.20	$ 0.22	$ 0.45	$ 0.95	$ 0.98
Diluted (loss) earnings per share of common stock from Discontinued Operations					$ 0.03	$ 0.11	$ 0.07	$ 0.08	$ (1.31)	$ 0.29	$ 0.16
Diluted (loss) earnings per share of common stock					$ 0.18	$ 0.49	$ 0.27	$ 0.30	$ (0.86)	$ 1.24	$ 1.14
Parent Company [Member]
Operating Revenue
Operating Expenses
Other operation and maintenance									1	1	1
Total operating expenses									1	1	1
Operating (Loss) Income									(1)	(1)	(1)
Other Income (Expenses)
Interest expense									(42)	(33)	(29)
Income from equity investments									204	237	243
Impairment Losses											(5)
Total Other Expenses									162	204	209
Income from Continuing Operations Before Income Tax									161	203	208
Income Tax Expense (Benefit) Related to Continuing Operations									51	(15)	(14)
Net Income from Continuing Operations									110	218	222
(Loss)Income from Discontinued Operations, net of Income Taxes									(322)	67	35
Net (Loss) Income									(212)	285	257
Comprehensive (Loss) Income									$ (198)	$ 300	$ 300
Earnings Per Share
Basic earnings per share of common stock from continuing operations									$ 0.45	$ 0.95	$ 0.98
Basic (loss) earnings per share of common stock from Discontinued Operations									$ (1.31)	$ 0.30	$ 0.16
Basic (loss) earnings per share of common stock									$ (0.86)	$ 1.25	$ 1.14
Diluted earnings per share of common stock from continuing operations									$ 0.45	$ 0.95	$ 0.98
Diluted (loss) earnings per share of common stock from Discontinued Operations									$ (1.31)	$ 0.29	$ 0.16
Diluted (loss) earnings per share of common stock									$ (0.86)	$ 1.24	$ 1.14