Benefit Obligation Activity Using an Actuarial Measurement Date for Qualified Plan and SERP (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Change in projected benefit obligation:
Benefit obligation at beginning of year	$ 245	$ 221
Service cost	11	10	10
Interest cost	14	14	12
Actuarial loss	30	6
Benefits paid	(19)	(6)
Projected benefit obligation at end of year	281	245	221
Accumulated benefit obligation	236	213
Change in plan assets:
Fair value of plan assets at beginning of year	183	144
Actual return on plan assets	17	20
Employer contributions	35	25
Benefits paid	(19)	(6)
Fair value of plan assets at end of year	216	183	144
Unfunded status:
Unfunded status at end of year	$ (65)	$ (62)