Associate Benefit Plans - Additional Information (Detail)		3 Months Ended	12 Months Ended
	Mar. 31, 2014	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) plan shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to the plan on behalf of associates, cash expenditure			$ 6,600,000	$ 5,800,000	$ 3,800,000
Percentage of contributions to be invested in balanced fund if no designation made (as percent)			100.00%
Employee benefit plan, sales of common stock (in shares) | shares			14,000	9,000	51,000
Employee benefit plan purchase of common stock (in shares) | shares			0	0
Requisite service period	10 years
Amortization of unrecognized gains losses exceed (as percent)			10.00%
Percentage of annual medical premium cap (as percent)			4.00%
Amount of annual health premium per retiree			$ 3,843
Expected future employer contributions, per employee				$ 3,997
During 2021			$ 55,000
Asset allocation percent			100.00%	100.00%
Number of additional supplemental plans | plan			4
Less than 100%
Defined Benefit Plan Disclosure [Line Items]
Target allocation percent			110.00%
More than 100%
Defined Benefit Plan Disclosure [Line Items]
Target allocation percent			110.00%
Maximum
Defined Benefit Plan Disclosure [Line Items]
Average annual rate of increase for medical benefits (less than) (as percent)			10.00%
Supplemental Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)			$ 500,000	$ 600,000
Early Retirement Window Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)			100,000	100,000
Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)			1,400,000	1,400,000
Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)			$ 100,000	$ 100,000
Risk Management
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent			65.80%	62.70%
Risk Management | Minimum | Less than 100%
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent			40.00%
Risk Management | Minimum | More than 100%
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent			50.00%
Risk Management | Maximum | Less than 100%
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent			80.00%
Risk Management | Maximum | More than 100%
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent			90.00%
Return Enhancement | Minimum | Less than 100%
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent			20.00%
Return Enhancement | Minimum | More than 100%
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent			10.00%
Return Enhancement | Maximum | Less than 100%
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent			60.00%
Return Enhancement | Maximum | More than 100%
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent			50.00%
Alliance | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Amount contributed to plan to settle the obligation		$ 500,000
Beneficial | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
During 2021			$ 4,694,000