Postretirement Plans - Additional Information (Detail) (USD $) Share data in Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	$ 32,888,000,000	$ 29,041,000,000
Aggregate liability for postemployment plans	109,000,000	107,000,000
Actuarial losses and prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2013	1,600,000,000
Actuarial losses and prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2013, net of tax	1,000,000,000
Expected pension plan expenses during 2013	1,900,000,000
Contributions made to defined contribution plans	380,000,000	378,000,000	379,000,000
Shares of our stock held in defined contribution plans we sponsor	48.6	52.1
Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for defined benefit pension plans	40,400,000,000	35,700,000,000
Accumulated benefit obligation in excess of plan assets	40,200,000,000	35,500,000,000
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	30,924,000,000	27,292,000,000	25,345,000,000
Unrecognized net actuarial losses	19,121,000,000	15,698,000,000
Actuarial losses and prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2013	1,500,000,000
Actuarial losses and prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2013, net of tax	963,000,000
Increase in projected benefit obligations due to a decrease in discount rate	4,500,000,000	3,800,000,000
Contributions made by the employer	3,637,000,000	2,285,000,000
Planned defined benefit plan contributions by employer in next fiscal year	1,500,000,000
Non-Qualified Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, liabilities	1,000,000,000	907,000,000
Unrecognized net actuarial losses	544,000,000	476,000,000
Benefit plan expense	107,000,000	104,000,000	85,000,000
Separate Trust
Defined Benefit Plan Disclosure [Line Items]
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	270,000,000	283,000,000
Retiree Medical And Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans	1,964,000,000	1,749,000,000	1,833,000,000
Unrecognized net actuarial losses	777,000,000	648,000,000
Contributions made by the employer	$ 235,000,000