Associate Benefit Plans - Additional Information (Detail)		12 Months Ended
	Mar. 31, 2014	Dec. 31, 2019 USD ($) plan shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to the plan on behalf of associates, cash expenditure		$ 5,800,000	$ 3,800,000	$ 3,600,000
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		9,000	51,000	156,000
Employee benefit plan purchase of common stock (in shares) | shares		0	0	83,000
Requisite service period	10 years
Amortization of unrecognized gains losses exceed (as percent)		10.00%
Expected amortization of net actuarial gain in next fiscal year		$ 100,000
Expected amortization of net transition obligation in next fiscal year		$ 100,000
Percentage of annual medical premium cap (as percent)		4.00%
Amount of annual health premium per retiree		$ 3,695
Expected future employer contributions, per employee			$ 3,843
Defined benefit plan fair value of assets		$ 0	0	$ 0	$ 0
Number of additional supplemental plans | plan		4
Asset allocation percent		100.00%
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%
Risk Management
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent		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%
Supplemental Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)		$ 600,000	700,000
Early Retirement Window Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)		100,000	100,000
Director's Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding asset (less than)			100,000
Supplemental Executive Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)		1,400,000	1,500,000
Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)		100,000	100,000
Alliance
Defined Benefit Plan Disclosure [Line Items]
Expected amortization of net actuarial gain in next fiscal year		300,000
Beneficial
Defined Benefit Plan Disclosure [Line Items]
Expected amortization of net actuarial gain in next fiscal year		(100,000)
Pennsylvania | Alliance
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets		7,431,000	$ 6,533,000	$ 8,378,000	$ 7,504,000
Net amount recognized		$ 200,000