Components of Net Periodic Costs for Qualified and Supplemental Pension Plans (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
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3	$ 3	$ 9	$ 8
Interest cost	4	4	11	11
Expected return on plan assets	(4)	(4)	(12)	(12)
Amortization of actuarial loss and prior service cost	2	1	5	3
Net periodic benefit cost	$ 5	$ 4	$ 13	$ 10