Retirement Plans - Weighted Average Assumptions Used to Determine Benefit Obligation (Detail)	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Ultimate	0.00%	0.00%
Discount rate	3.40%	3.80%
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Ultimate	4.50%	5.00%
Discount rate	3.50%	4.00%
Pre-Age 65 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	0.00%	0.00%
Pre-Age 65 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	6.30%	6.10%
Post-Age 65 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	0.00%	0.00%
Post-Age 65 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	6.30%	6.10%