Employee Bargaining Agreements and Benefit Plans - Assumption Rates Used for Net Periodic Benefit Costs (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Discount rate:			2.50%
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate:	2.00%	2.65%
Expected return on assets:	2.54%	3.02%	3.10%
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate:	2.10%	2.90%
Expected return on assets:	3.03%	3.62%	4.17%