Amounts Recognized for Pension and Postretirement Benefits in Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Amounts recognized for pension and postretirement benefits in other comprehensive income
Amortization of prior service cost included in net periodic pension and postretirement cost, Net-of-tax Amount	$ (2)	$ (1)	$ (4)	$ (3)
Amortization of actuarial loss included in net periodic pension and postretirement cost, Net-of-tax Amount	5	5	15	11
Other comprehensive income - pension benefits, Net-of-tax Amount	3	4	11	8
Amortization of prior service cost included in net periodic pension and postretirement cost, Net-of-tax Amount	(2)	(1)	(4)	(3)
Amortization of prior service cost included in net periodic pension and postretirement cost, Tax Benefit
Amortization of actuarial loss included in net periodic pension and postretirement cost, Net-of-tax Amount	5	5	15	11
Amortization of actuarial loss included in net periodic pension and postretirement cost, Tax Benefit	1	2		1
Other comprehensive income - pension benefits, Net-of-tax Amount	3	4	11	8
Other comprehensive - pension benefits, Tax Benefit	1	2		1
Amortization of prior service cost included in net periodic pension and postretirement cost, Before-tax amount	(2)	(1)	(4)	(3)
Amortization of actuarial loss included in net periodic pension and postretirement cost, Before-tax amount	4	3	15	10
Other comprehensive income - pension benefits, Before-tax amount	$ 2	$ 2	$ 11	$ 7