Debt and Credit Arrangements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Times	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Long Term Debt	$ 3,300	$ 3,300
Interest rate of capital securities through April 14, 2017	6.375%
Interest rate on capital securities after April 14, 2017, plus three-month LIBOR rate	2.25%
Consecutive years payment of interest may be deferred on capital securities	10 years
End date of restrictions to redeem capital securities	Mar. 29, 2047
Capital securities redeemable date	Apr. 15, 2017
Interest costs incurred	209	213	224
Interest paid	205	213	220
Maximum unsecured borrowings permitted under credit agreement	500	500
Revolving credit agreement maturity date	Sep. 24, 2017
Number of times permitted for increase in available credit, per year	2
Maximum facility amount permitted to request an increase in the credit available	750
Extended additional maturity period	1 year
Borrowings under credit agreement	0
6.375% Capital Securities due March 29, 2067 [Member]
Debt Instrument [Line Items]
Long Term Debt	1,000	1,000
Scheduled maturity date	Apr. 15, 2037
Final maturity date	Mar. 29, 2067
Interest rate of capital securities through April 14, 2017	6.375%
Debt instrument maturity date	Mar. 29, 2067
6.8% Debentures due November 15, 2031 [Member]
Debt Instrument [Line Items]
Long Term Debt	$ 200	$ 200
Interest rate due on notes and debentures	6.80%
Debt instrument maturity date	Nov. 15, 2031