Employee Benefits - Additional Information (Detail) Employee in Thousands		3 Months Ended		6 Months Ended
	Aug. 15, 2018 Employee	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Amount paid in severance benefits over the period		$ 100,000,000		$ 300,000,000
Severance charges		339,000,000	$ 195,000,000	339,000,000	$ 195,000,000
Post employment benefits liability		600,000,000		600,000,000
Forecast | Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Collective bargaining agreements term	4 years
Collective bargaining agreements, number of employees covered | Employee	34
Pension | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Employer discretionary contribution amount				1,000,000,000	$ 3,400,000,000
Pension | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan contributions by employer		0		$ 0
Health Care and Life
Defined Benefit Plan Disclosure [Line Items]
Employer discretionary contribution amount		$ 700,000,000