Pension Plans, Other Postretirement Benefits, and Defined-Contribution Plans - Estimated Future Contributions and Current Year Contributions Table (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions in 2017	$ 152
Employer contributions	276
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	254	$ 101
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	22	$ 19
Funded Plans [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions in 2017	114
Employer contributions	167
Unfunded Plans [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions in 2017	16
Employer contributions	87
Unfunded Plans [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions in 2017	22
Employer contributions	$ 22