Pension and Other Postretirement Benefits - Schedule of Fair Value of Pension Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net receivables which represents assets related to divested businesses to be transferred to the buyers	$ 28	$ 20
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	4,862	4,666	$ 10,928
Fair Value, Measurements, Recurring | Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	4,834	4,646
Defined benefit plan, alternative investments, fair value of plan asset	2,932	2,878
Fair Value, Measurements, Recurring | Pension benefits | Equities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,357	1,395
Defined benefit plan, alternative investments, fair value of plan asset	978	1,002
Fair Value, Measurements, Recurring | Pension benefits | Equities | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	972	824
Defined benefit plan, alternative investments, fair value of plan asset	593	431
Fair Value, Measurements, Recurring | Pension benefits | Equities | Hedge Funds, Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	230	406
Defined benefit plan, alternative investments, fair value of plan asset	230	406
Fair Value, Measurements, Recurring | Pension benefits | Equities | Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	155	165
Defined benefit plan, alternative investments, fair value of plan asset	155	165
Fair Value, Measurements, Recurring | Pension benefits | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,278	1,958
Defined benefit plan, alternative investments, fair value of plan asset	924	729
Fair Value, Measurements, Recurring | Pension benefits | Fixed income | Intermediate and Long Duration Government Credit
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,961	773
Defined benefit plan, alternative investments, fair value of plan asset	779	655
Fair Value, Measurements, Recurring | Pension benefits | Fixed income | Other Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	317	1,185
Defined benefit plan, alternative investments, fair value of plan asset	145	74
Fair Value, Measurements, Recurring | Pension benefits | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,199	1,293
Defined benefit plan, alternative investments, fair value of plan asset	1,030	1,147
Fair Value, Measurements, Recurring | Pension benefits | Other investments | Other Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	359	243
Defined benefit plan, alternative investments, fair value of plan asset	275	178
Fair Value, Measurements, Recurring | Pension benefits | Other investments | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	257	266
Defined benefit plan, alternative investments, fair value of plan asset	172	185
Fair Value, Measurements, Recurring | Pension benefits | Other investments | Discretionary And Systematic Macro Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	583	784
Defined benefit plan, alternative investments, fair value of plan asset	583	784
Fair Value, Measurements, Recurring | Pension benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	906	1,622
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Equities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	379	393
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Equities | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	379	393
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Equities | Hedge Funds, Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	0	0
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Equities | Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	0	0
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	365	1,083
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Fixed income | Intermediate and Long Duration Government Credit
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	201	23
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Fixed income | Other Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	164	1,060
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	162	146
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Other investments | Other Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	77	65
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Other investments | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	85	81
Fair Value, Measurements, Recurring | Pension benefits | Level 1 | Other investments | Discretionary And Systematic Macro Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	0	0
Fair Value, Measurements, Recurring | Pension benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	996	146
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Equities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	0	0
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Equities | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	0	0
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Equities | Hedge Funds, Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	0	0
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Equities | Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	0	0
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	989	146
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Fixed income | Intermediate and Long Duration Government Credit
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	981	95
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Fixed income | Other Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	8	51
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	7	0
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Other investments | Other Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	7	0
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Other investments | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	0	0
Fair Value, Measurements, Recurring | Pension benefits | Level 2 | Other investments | Discretionary And Systematic Macro Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	$ 0	$ 0