Employee Retirement Benefits - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008 Year
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Minimum participant age to elect				40
Payment for terminated vested participants	$ 19	$ 6
Matching contributions for 401(k) plan	8	8	7
Number of years of eligible service for vesting	3 years
Pension plan assumptions	The change in the amount recognized in other comprehensive income is largely due to the decrease in discount rate. We assume that 100 percent of participants will choose lump sum payments.
Total recognized in net periodic benefit cost and other comprehensive income	38	9	25
Amortization of actuarial loss	6
Amortization of prior service cost	1
Pension plan assets at fair value	216	183	144
Cash dividends paid	261	259	255
Domestic Equity Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Allocation percentage of equity investments	70.00%
Domestic Equity Securities | Information technology sector
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Allocation percentage of equity investments	20.00%
Domestic Equity Securities | Financial services sector
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Allocation percentage of equity investments	19.00%
Domestic Equity Securities | Healthcare sector
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Allocation percentage of equity investments	13.00%
Domestic Equity Securities | All remaining sectors
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Allocation percentage of equity investments	10.00%
US States and Political Subdivisions Debt Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Allocation percentage of fixed-maturity investments	16.00%
Domestic Corporate Debt Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Allocation percentage of fixed-maturity investments	12.00%
Preferred Stock
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Allocation percentage of equity investments	2.00%
Cash and Cash Equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension plan assets at fair value	2
Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Matching contribution to defined contribution plan	6.00%
Qualified Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension plan assets, shares	567,113	642,113
Pension plan assets, fair value	17	20
Pension plan assets, sold shares	75,000	0
Pension plan assets, realized gain	2
Cash dividends paid	1	1
Expected contribution by employer during 2012	14
Qualified Pension Plan | Terminated Vested Participants
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Payment for terminated vested participants	9
Qualified Pension Plan | Benefit Obligations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate decreased by	0.75%
Qualified Pension Plan | Net Periodic Benefit Costs
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate decreased by	0.25%
Supplemental Executive Retirement Plan (SERP)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accrued Employee Benefit	8	6
Employee Benefit Liability			1
Matching Deferrals to the Top Hat Savings Plan			6.00%
Expected contribution by employer during 2012	$ 2
Supplemental Executive Retirement Plan (SERP) | Benefit Obligations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate decreased by	0.80%
Supplemental Executive Retirement Plan (SERP) | Net Periodic Benefit Costs
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate decreased by	0.55%