Postretirement Benefits Other Than Pensions - Summary of Assumptions Used to Determine Benefit Obligations (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Benefit Obligation
Defined Benefit Plan Disclosure [Line Items]
Healthcare cost trend rate assumed for next year (as a percent)	6.72%	7.00%
Assumed decline in cost trend rate (as a percent)	4.50%	4.50%
Year of ultimate rate achievement	2038	2038
Weighted average discount rate (as a percent)	3.53%	3.97%
Net Periodic Benefit Cost
Defined Benefit Plan Disclosure [Line Items]
Healthcare cost trend rate assumed for next year (as a percent)	7.00%	7.27%	7.70%
Assumed decline in cost trend rate (as a percent)	4.50%	4.50%	4.50%
Year of ultimate rate achievement	2038	2038	2029
Weighted average discount rate (as a percent)	3.97%	4.27%	3.85%
Effective rate for interest on benefit obligations (as a percent)	3.32%	3.52%	3.85%
Effective discount rate for service cost (as a percent)	4.44%	4.68%	3.85%
Effective rate for interest on service cost (as a percent)	4.08%	4.37%	3.85%