Retirement Plans - Weighted Average Assumptions Used to Determine Benefit Obligation (Detail)	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Ultimate	0.00%	0.00%
Discount rate	2.40%	3.00%
Interest crediting rate for cash balance plans	2.40%	3.40%
OPEB
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend, Ultimate	4.50%	4.50%
Discount rate	2.20%	3.00%
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	6.20%	6.20%
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	6.20%	6.20%