Reconciliation of Change in Projected Benefit Obligation and Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation:	$ 4,739	$ 4,158
Benefit obligation at beginning of year	5,253	4,473
Service cost	195	112	$ 124
Interest cost	220	200	200
Actuarial (gain) loss	525	738
Gross benefits paid and settlements	(366)	(243)
Other	25	(27)
Benefit obligation at end of year	5,852	5,253	4,473
Fair value of plan assets at beginning of year	3,355	2,975
Actual return on plan assets	510	230
Employer contributions	419	421
Gross benefits paid and settlements	(366)	(243)
Other	14	(28)
Fair value of plan assets at end of year	3,932	3,355	2,975
Funded status-Net amount recognized	(1,920)	(1,898)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	1,687	2,002
Service cost	13	19	21
Interest cost	66	86	82
Plan participants' contributions	68	69
Benefits paid	(178)	(191)
Actuarial (gain) loss	40	(165)
Plan amendments		(138)
Other	14	5
Benefit obligation at end of year	1,710	1,687	2,002
Fair value of plan assets at beginning of year	55	56
Actual return on plan assets	1	2
Employer contributions	108	119
Plan participants' contributions	68	69
Benefits paid	(178)	(191)
Fair value of plan assets at end of year	54	55	$ 56
Funded status-Net amount recognized	$ (1,656)	$ (1,632)