Retirement Plans - Weighted Average Assumptions Used to Determine Benefit Obligation (Detail)	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Ultimate	0.00%	0.00%
Discount rate	4.00%	3.40%
OPEB
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Ultimate	4.50%	4.50%
Discount rate	4.20%	3.50%
Pre-Age 65 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	0.00%	0.00%
Pre-Age 65 | OPEB
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	6.50%	6.30%
Post-Age 65 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	0.00%	0.00%
Post-Age 65 | OPEB
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Current	6.50%	6.30%