Pensions and Other Post-retirement Benefits - Schedule of Assumptions Used to Determine Benefit Obligations and Net Periodic Benefit Cost (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Assumptions used to determine benefit obligations, average discount rate	2.28%	2.86%
Assumptions used to determine benefit obligations, rate of compensation increase	2.93%	2.93%
Assumptions used to determine net periodic benefit cost, average discount rate, service cost	3.08%	3.10%
Assumptions used to determine net periodic benefit cost, average discount rate, interest cost	2.52%	2.52%
Assumptions used to determine net periodic benefit cost, expected return on plan assets	7.10%	7.09%
Assumptions used to determine net periodic benefit cost, rate of compensation increases	2.93%	2.93%
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Assumptions used to determine benefit obligations, average discount rate	2.21%	3.05%
Assumptions used to determine benefit obligations, rate of compensation increase	3.00%	0.00%
Assumptions used to determine net periodic benefit cost, average discount rate, service cost	2.42%	3.15%
Assumptions used to determine net periodic benefit cost, average discount rate, interest cost	1.48%	2.61%
Assumptions used to determine net periodic benefit cost, expected return on plan assets	0.00%	0.00%
Assumptions used to determine net periodic benefit cost, rate of compensation increases	3.00%	0.00%