Employee Postretirement Benefits - Schedule of Assumptions Used to Determine Benefit (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.33%	3.62%
Rate of compensation increase-weighted average	3.75%	3.32%
Discount rate	3.62%	4.11%	4.69%
Expected long-term return on plan assets	6.85%	7.00%	7.00%
Rate of compensation increase	3.32%	2.57%	2.59%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.38%	3.70%
Rate of compensation increase-weighted average	3.75%	3.31%
Discount rate	3.70%	4.28%
Rate of compensation increase	3.31%	2.48%	2.50%
Healthcare cost trend rate
Ultimate rate	4.50%	4.50%	4.50%
Year rate reaches ultimate	2038	2038	2038
Other Postretirement Benefits [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate			4.667%
Other Postretirement Benefits [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate			3.85%
Other Postretirement Benefits [Member] | Immediate Rate [Member]
Healthcare cost trend rate
Immediate rate	6.31%	6.58%
Other Postretirement Benefits [Member] | Initial Rate [Member]
Healthcare cost trend rate
Immediate rate	6.58%	6.83%	7.05%