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, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions in 2016	$ 46
Employer contributions	58
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	43	$ 121
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	15	$ 15
Funded Plans Defined Benefit [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions in 2016	5
Employer contributions	4
Unfunded Plans Defined Benefit [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions in 2016	25
Employer contributions	39
Unfunded Plans Defined Benefit [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions in 2016	16
Employer contributions	$ 15