Employee Postretirement Benefits - Schedule of Assumptions Used to Determine Benefit (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.62%	4.11%
Rate of compensation increase-weighted average	3.32%	2.57%
Discount rate	4.11%	4.688%	4.258%
Expected long-term return on plan assets	7.00%	7.00%	7.00%
Rate of compensation increase	2.57%	2.59%	3.87%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.70%	4.28%
Rate of compensation increase-weighted average	3.31%	2.48%
Healthcare cost trend rate
Immediate rate	6.58%	6.83%
Ultimate rate	4.50%	4.50%
Year rate reaches ultimate	2038	2038
Other Postretirement Benefits [Member] | Net Periodic Benefit Cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.28%		4.206%
Rate of compensation increase	2.48%	2.50%	3.86%
Healthcare cost trend rate
Immediate rate	6.83%	7.05%	7.00%
Ultimate rate	4.50%	4.50%	4.50%
Year rate reaches ultimate	2038	2038	2025
Other Postretirement Benefits [Member] | Net Periodic Benefit Cost [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.667%
Other Postretirement Benefits [Member] | Net Periodic Benefit Cost [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.85%