Schedule I - Condensed Financial Information of Parent Company (Statements of Cash Flows) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
OPERATING ACTIVITIES
Net Income (loss)	$ 242	$ (212)	$ 285
Loss (income) from discontinued operations, net of income taxes		322	(67)
Adjustments to reconcile net income to net cash from operating activities:
Deferred income taxes	302	458	312
Other	3	(13)	(15)
Changes in:
Other assets and liabilities	10	9	16
FINANCING ACTIVITIES
Dividends paid on common stock	(272)	(270)	(248)
Common stock issued for the Direct Stock Purchase and Dividend Reinvestment Plan and employee-related compensation	34	50	51
Issuances of common stock		324
Issuances of Series A preferred stock	126
Issuances (repayments) of short-term debt, net	164	(200)	33
Issuance of term loan		250	200
Repayments of term loans	(100)	(450)
Costs of issuances	(10)	(23)	(9)
Net Increase (Decrease) In Cash and Cash Equivalents	(9)	(2)	(84)
Cash and Cash Equivalents at Beginning of Year	23	25	109
CASH AND CASH EQUIVALENTS AT END OF YEAR	14	23	25
Parent Company [Member]
OPERATING ACTIVITIES
Net Income (loss)	242	(212)	285
Loss (income) from discontinued operations, net of income taxes		322	(67)
Adjustments to reconcile net income to net cash from operating activities:
Distributions from related parties less than earnings	(149)	(127)	(52)
Deferred income taxes	(5)	(7)	(31)
Other	18
Changes in:
Prepaid and other	13	2	(23)
Accounts payable	1	6	6
Interest and taxes		(141)	39
Other assets and liabilities	1	3	4
Net Cash From (Used By) Operating Activities	121	(154)	161
FINANCING ACTIVITIES
Dividends paid on common stock	(272)	(270)	(248)
Common stock issued for the Direct Stock Purchase and Dividend Reinvestment Plan and employee-related compensation	34	50	51
Issuances of common stock		324
Issuances of Series A preferred stock	126
Capital contributions to subsidiaries, net	(210)	(250)	(110)
Decrease in notes receivable from associated companies			154
Increase in notes payable due to associated companies	3	491
Issuances (repayments) of short-term debt, net	263	(240)	(201)
Issuance of term loan		250	200
Repayments of term loans		(450)
Costs of issuances		(13)	(2)
Net Cash Used By Financing Activities	(56)	(108)	(156)
Net Increase (Decrease) In Cash and Cash Equivalents	65	(262)	5
Cash and Cash Equivalents at Beginning of Year		262	257
CASH AND CASH EQUIVALENTS AT END OF YEAR	$ 65		$ 262