Schedule I - Condensed Financial Information of Parent Company (Statements of Cash Flows) (Detail) - USD ($) $ 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
OPERATING ACTIVITIES
Net Income (Loss)	$ 130	$ 91	$ 53	$ 53	$ 35	$ 79	$ 53	$ 75	$ 327	$ 242	$ (212)
(Income) loss from discontinued operations, net of income taxes	(9)								(9)		322
Adjustments to reconcile net income to net cash from operating activities:
Deferred income taxes									122	302	458
Increase in fair value of preferred stock derivative		$ (15)							(15)
Other									(2)	(6)	(13)
Changes in:
Other assets and liabilities									8	10	9
Net Cash From (Used By) Operating Activities									939	854	497
FINANCING ACTIVITIES
Dividends paid on common stock									(275)	(272)	(270)
Common stock issued for the Direct Stock Purchase and Dividend Reinvestment Plan and employee-related compensation									18	34	50
Issuances of common stock											324
Issuances of Series A preferred stock									54	126
Reacquisitions of long-term debt									(420)	(334)	(558)
Issuances (repayments) of short-term debt, net									34	164	(200)
Borrowings under term loans									300		250
Repayments of term loans										(100)	(450)
Costs of issuances									(7)	(10)	(23)
Net Cash From (Used By) Financing Activities									233	363	(88)
Net Increase (Decrease) In Cash and Cash Equivalents									11	(9)	(2)
Cash and Cash Equivalents at Beginning of Year				14				$ 23	14	23	25
CASH AND CASH EQUIVALENTS AT END OF YEAR	$ 25				14				25	14	23
Parent Company [Member]
OPERATING ACTIVITIES
Net Income (Loss)									327	242	(212)
(Income) loss from discontinued operations, net of income taxes									(9)		322
Adjustments to reconcile net income to net cash from operating activities:
Distributions from related parties less than earnings									(74)	(149)	(127)
Deferred income taxes									(33)	(5)	(7)
Increase in fair value of preferred stock derivative									(15)
Other									13	18
Changes in:
Prepaid and other									6	13	2
Accounts payable									(1)	1	6
Interest and taxes									(23)		(141)
Other assets and liabilities									(22)	1	3
Net Cash From (Used By) Operating Activities									169	121	(154)
FINANCING ACTIVITIES
Dividends paid on common stock									(275)	(272)	(270)
Common stock issued for the Direct Stock Purchase and Dividend Reinvestment Plan and employee-related compensation									18	34	50
Issuances of common stock											324
Issuances of Series A preferred stock									54	126
Capital contributions to subsidiaries, net									(282)	(210)	(250)
Increase in notes payable due to associated companies									4	3	491
Reacquisitions of long-term debt									(250)
Issuances (repayments) of short-term debt, net									197	263	(240)
Borrowings under term loans									300		250
Repayments of term loans											(450)
Costs of issuances											(13)
Net Cash From (Used By) Financing Activities									(234)	(56)	(108)
Net Increase (Decrease) In Cash and Cash Equivalents									(65)	65	(262)
Cash and Cash Equivalents at Beginning of Year				$ 65					$ 65		$ 262
CASH AND CASH EQUIVALENTS AT END OF YEAR					$ 65					$ 65