Expected Cash Flows for the Pension Benefit and Other Postretirement Benefit Plans (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Pension Plan [Member]
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	$ 210	[1]
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	172	[1]
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	172	[1]
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	170	[1]
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	167	[1]
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	824	[1]
Pension Plan [Member] | Plan Trusts [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	0	[1]
Pension Plan [Member] | Plan Participants
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	9	[1],[2]
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months	7	[3]
Defined Benefit Plan, Expected Future Benefit Payment, Year Two	6	[3]
Defined Benefit Plan, Expected Future Benefit Payment, Year Three	6	[3]
Defined Benefit Plan, Expected Future Benefit Payment, Year Four	6	[3]
Defined Benefit Plan, Expected Future Benefit Payment, Year Five	5	[3]
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter	18	[3]
Other Postretirement Benefits Plan [Member] | Plan Trusts [Member]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	0	[3]
Other Postretirement Benefits Plan [Member] | Plan Participants
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	$ 0	[2],[3]

[1] Based on the funding status and ERISA limitations, the Company anticipates contributions to the Retirement Plan will not be required during 2018.

[2] The expected benefit payments for the SERP will be paid directly from the Company's corporate assets.

[3] Expected payments under other postretirement benefit plans are shown net of participant contributions.