Schedule I - Condensed Financial Information of Parent Company (Balance Sheets) (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CURRENT ASSETS
Cash and cash equivalents	$ 25	$ 14	$ 23	$ 25
Total Current Assets	1,202	1,034
INVESTMENTS AND OTHER ASSETS
Goodwill	1,406	1,407
Other	129	121
TOTAL ASSETS	16,326	15,589	14,778
CURRENT LIABILITIES
Short-term debt	1,063	729
Current portion of long-term debt	294	366
Total Current Liabilities	2,308	2,056
DEFERRED CREDITS
Long-term debt	$ 4,656	$ 4,397
COMMITMENTS AND CONTINGENCIES (NOTE 4)
PREFERRED STOCK
Series A preferred stock, $.01 par value, 18,000 shares authorized, 18,000 and 12,600 shares outstanding, respectively	$ 183	$ 129
EQUITY
Common stock, $.01 par value - 400,000,000 shares authorized, 254,289,261 and 252,728,684 shares outstanding, respectively	3	3
Premium on stock and other capital contributions	3,829	3,800
Accumulated other comprehensive loss	(36)	(46)	$ (34)	(48)
Retained earnings	617	565
TOTAL LIABILITIES AND EQUITY	16,326	15,589
Parent Company [Member]
CURRENT ASSETS
Cash and cash equivalents		65		$ 262
Prepayments of income taxes	171	152
Accounts receivable and other	18	9
Total Current Assets	189	226
INVESTMENTS AND OTHER ASSETS
Goodwill	1,398	1,398
Investment in consolidated companies	4,636	4,256
Other	127	82
Total Investments and Other Assets	6,161	5,736
TOTAL ASSETS	6,350	5,962
CURRENT LIABILITIES
Short-term debt	784	287
Current portion of long-term debt	190	250
Interest and taxes accrued	10	9
Accounts payable due to associated companies	8	13
Total Current Liabilities	992	559
DEFERRED CREDITS
Notes payable due to subsidiary companies	498	494
Liabilities and accrued interest related to uncertain tax positions		4
Total Deferred Credits	498	498
Long-term debt	$ 264	$ 454
COMMITMENTS AND CONTINGENCIES (NOTE 4)
PREFERRED STOCK
Series A preferred stock, $.01 par value, 18,000 shares authorized, 18,000 and 12,600 shares outstanding, respectively	$ 183	$ 129
EQUITY
Common stock, $.01 par value - 400,000,000 shares authorized, 254,289,261 and 252,728,684 shares outstanding, respectively	3	3
Premium on stock and other capital contributions	3,829	3,800
Accumulated other comprehensive loss	(36)	(46)
Retained earnings	617	565
Total Equity	4,413	4,322
TOTAL LIABILITIES AND EQUITY	$ 6,350	$ 5,962