Employee Benefits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for the pension plans	$ 2,479	$ 2,120
Estimated aggregate net actuarial loss and prior service cost to be amortized next year	95
Effect of one percent increase in health care cost trend rate on the accumulated other postretirement benefit obligation	83
Effect of one percent decrease in health care cost trend rate on the accumulated other postretirement benefit obligation	67
Effect of one percent increase in health care cost trend rate on net other postretirement benefit cost	6
Effect of one percent decrease in health care cost trend rate on net other postretirement benefit cost	5
Employer annual elective matching contribution related to employee's eligible pre-tax contributions	Equal to 100%
The portion of employer annual elective matching contribution related to employee's eligible pre-tax contributions	Up to 4%
Employer contributions for Capital Accumulation Plan	26	27	28
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	98	94
Fair value of plan assets invested in pooled funds	1,200	1,073
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	10	10
Fair value of plan assets invested in pooled funds	$ 96	$ 73
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Minimum range of portfolio allocation to equity securities	55.00%
Maximum range of portfolio allocation to equity securities	65.00%