Schedule I - Condensed Financial Information of Parent Company (Statements of Income (Loss)) (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Income Statements, Captions [Line Items]
Operating Revenue	$ 1,150	$ 1,362	$ 1,140	$ 1,371	$ 1,118	$ 1,313	$ 1,117	$ 1,330	$ 5,023	$ 4,878	$ 4,666
Operating Expenses
Other operation and maintenance									1,016	924	851
Total operating expenses	941	1,179	1,001	1,229	1,004	1,147	966	1,157	4,350	4,274	3,998
Operating Income (Loss)	209	183	139	142	114	166	151	173	673	604	668
Other Income (Expenses)
Interest expense									(280)	(268)	(273)
Income from equity investments											2
Other income									54	44	32
Total Other Income (Expenses)	(65)	(43)	(59)	(59)	(66)	(53)	(53)	(52)	(226)	(224)	(239)
Income from Continuing Operations Before Income Tax	144	140	80	83	48	113	98	121	447	380	429
Income Tax Expense Related to Continuing Operations	23	49	27	30	13	34	45	46	129	138	319
Net Income from Continuing Operations	121	91	53	53					318	242	110
Income (Loss) from Discontinued Operations, net of Income Taxes	9								9		(322)
Net Income (Loss)	$ 130	$ 91	$ 53	$ 53	$ 35	$ 79	$ 53	$ 75	327	242	(212)
Comprehensive Income (Loss)									$ 337	$ 230	$ (198)
Earnings Per Share
Basic earnings per share of common stock from continuing operations									$ 1.25	$ 0.96	$ 0.45
Basic earnings (loss) per share of common stock from discontinued operations									0.04		(1.31)
Basic earnings (loss) per share of common stock									1.29	0.96	(0.86)
Diluted earnings per share of common stock from continuing operations									1.25	0.96	0.45
Diluted earnings (loss) per share of common stock from discontinued operations									0.04		(1.31)
Diluted earnings (loss) per share of common stock									$ 1.29	$ 0.96	$ (0.86)
Parent Company [Member]
Condensed Income Statements, Captions [Line Items]
Operating Revenue									$ 0	$ 0	$ 0
Operating Expenses
Other operation and maintenance									20	31	1
Total operating expenses									20	31	1
Operating Income (Loss)									(20)	(31)	(1)
Other Income (Expenses)
Interest expense									(47)	(43)	(42)
Income from equity investments									312	291	204
Other income									15
Total Other Income (Expenses)									280	248	162
Income from Continuing Operations Before Income Tax									260	217	161
Income Tax Expense Related to Continuing Operations									(58)	(25)	51
Net Income from Continuing Operations									318	242	110
Income (Loss) from Discontinued Operations, net of Income Taxes									9		(322)
Net Income (Loss)									327	242	(212)
Comprehensive Income (Loss)									$ 337	$ 230	$ (198)
Earnings Per Share
Basic earnings per share of common stock from continuing operations									$ 1.25	$ 0.96	$ 0.45
Basic earnings (loss) per share of common stock from discontinued operations									0.04		(1.31)
Basic earnings (loss) per share of common stock									1.29	0.96	(0.86)
Diluted earnings per share of common stock from continuing operations									1.25	0.96	0.45
Diluted earnings (loss) per share of common stock from discontinued operations									0.04		(1.31)
Diluted earnings (loss) per share of common stock									$ 1.29	$ 0.96	$ (0.86)