Employee Postretirement Benefits - Schedule of Assumptions Used to Determine Benefit (Detail)			1 Months Ended		12 Months Ended
	Jan. 01, 2019	[1]	Oct. 31, 2019	[1]	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate					2.37%	3.21%
Rate of compensation increase					3.07%	3.79%
Discount rate					3.21%	4.33%	3.62%
Expected long-term return on plan assets	7.35%		7.00%		7.00%		6.85%
Rate of compensation increase					3.79%	3.75%	3.32%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate					2.47%	3.32%
Rate of compensation increase					3.07%	3.79%
Discount rate					3.32%	4.38%	3.70%
Rate of compensation increase					3.79%	3.75%	3.31%
Healthcare cost trend rate
Ultimate rate					4.50%	4.50%	4.50%
Year rate reaches ultimate					2038	2038	2038
Other Postretirement Benefits [Member] | Immediate Rate [Member]
Healthcare cost trend rate
Immediate rate					5.74%	6.03%
Other Postretirement Benefits [Member] | Initial Rate [Member]
Healthcare cost trend rate
Immediate rate					6.03%	6.31%	6.58%

[1] The expected return on assets was 7.35% as of January 1, 2019 and 7.00% as of October 31, 2019 when a plan remeasurement occurred as a result of a plan curtailment.