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Postretirement Plans - Additional Information (Detail) - USD ($)
shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
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 $ 33,909 $ 36,605  
Aggregate liability for postemployment plans 70 88  
Actuarial losses and net prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2016 1,100    
Actuarial losses and net prior service cost related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2016, net of tax 693    
Expected pension plan expenses during next fiscal year 1,000    
Contributions made to defined contribution plans $ 393 $ 385 $ 383
Shares of our stock held in defined contribution plans we sponsor 40.0 41.7  
Qualified defined benefit pension plans      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation for defined benefit pension plans $ 43,500 $ 45,200  
Accumulated benefit obligation in excess of plan assets 43,400 45,000  
Assets set aside expected to be used to pay obligations of defined benefit pension plans and retiree medical and life insurance plans 32,096 34,673 33,010
Unrecognized net actuarial losses 19,632 20,794  
Actuarial losses related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2016 1,000    
Actuarial losses related to postretirement benefit plans expected to be recognized in net periodic benefit cost during 2016, net of tax 629    
Increase (decrease) in projected benefit obligations due to a change in discount rate (2,100) 4,800  
Contributions made by the employer 5 2,000  
Contributions expected to made by employer in 2016 25    
Non-Qualified Pension Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit pension plan, liabilities 1,200 1,100  
Unrecognized net actuarial losses 632 662  
Benefit plan expense 117 115 $ 108
Unrecognized prior service credit 95 121  
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 $ 421 $ 397