Components of Net Periodic Costs for Qualified and Supplemental Pension Plans (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3	$ 2	$ 6	$ 5
Interest cost	4	4	7	7
Expected return on plan assets	(4)	(4)	(8)	(8)
Amortization of actuarial loss and prior service cost	1	1	3	2
Net periodic benefit cost	$ 4	$ 3	$ 8	$ 6