Employee Postretirement Benefits - Schedule of Assumptions Used to Determine Benefit (Parenthetical) (Detail)			1 Months Ended		12 Months Ended
	Jan. 01, 2019		Oct. 31, 2019		Dec. 31, 2020	Dec. 31, 2018
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected long-term return on plan assets	7.35%	[1]	7.00%	[1]	7.00%	6.85%

[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.