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Reconciliation of Change in Projected Benefit Obligation and Plan Assets (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation: $ 4,158 $ 3,795  
Benefit obligation at beginning of year 4,473 4,803  
Service cost 112 124 $ 98
Interest cost 200 200 201
Actuarial (gain) loss 738 (298)  
Gross benefits paid and settlements (243) (343)  
Other (27) (13)  
Benefit obligation at end of year 5,253 4,473 4,803
Fair value of plan assets at beginning of year 2,975 2,562  
Actual gain (loss) on plan assets 230 (59)  
Employer contributions 421 824  
Gross benefits paid and settlements (243) (343)  
Other (28) (9)  
Fair value of plan assets at end of year 3,355 2,975 2,562
Funded status-Net amount recognized (1,898) (1,498)  
Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation at beginning of year 2,002 2,052  
Service cost 19 21 19
Interest cost 86 82 88
Plan participants' contributions 69 68  
Benefits paid (191) (205)  
Actuarial (gain) loss (165) (22)  
Plan amendments (138)    
Other 5 6  
Benefit obligation at end of year 1,687 2,002 2,052
Fair value of plan assets at beginning of year 56 57  
Actual gain (loss) on plan assets 2 1  
Employer contributions 119 135  
Plan participants' contributions 69 68  
Benefits paid (191) (205)  
Fair value of plan assets at end of year 55 56 $ 57
Funded status-Net amount recognized $ (1,632) $ (1,946)