Associate Benefit Plans - Fair Value Weighted Average Asset Allocations (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0	$ 0	$ 0
Asset allocation percent	100.00%	100.00%
Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 111,129	$ 100,388
Quoted Prices in Active Markets for Identical Asset (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	111,129	100,388
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	0	0
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0
Large cap
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent	5.20%	4.50%
Large cap | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 5,827	$ 4,532
Large cap | Quoted Prices in Active Markets for Identical Asset (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	5,827	4,532
Large cap | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	0	0
Large cap | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0
Mid cap
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent	0.10%	0.10%
Mid cap | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 69	$ 67
Mid cap | Quoted Prices in Active Markets for Identical Asset (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	69	67
Mid cap | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	0	0
Mid cap | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0
Small cap
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent	0.10%	0.10%
Small cap | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 61	$ 63
Small cap | Quoted Prices in Active Markets for Identical Asset (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	61	63
Small cap | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	0	0
Small cap | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0
International
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent	9.20%	7.90%
International | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 10,249	$ 7,902
International | Quoted Prices in Active Markets for Identical Asset (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	10,249	7,902
International | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	0	0
International | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0
Global Managed Volatility
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent	8.10%	6.90%
Global Managed Volatility | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 8,970	$ 6,955
Global Managed Volatility | Quoted Prices in Active Markets for Identical Asset (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	8,970	6,955
Global Managed Volatility | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	0	0
Global Managed Volatility | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0
U.S. Managed Volatility
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent	3.00%	2.60%
U.S. Managed Volatility | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 3,363	$ 2,611
U.S. Managed Volatility | Quoted Prices in Active Markets for Identical Asset (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	3,363	2,611
U.S. Managed Volatility | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	0	0
U.S. Managed Volatility | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0
Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent	65.80%	62.70%
Fixed Income | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 73,071	$ 62,924
Fixed Income | Quoted Prices in Active Markets for Identical Asset (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	73,071	62,924
Fixed Income | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	0	0
Fixed Income | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0
U.S. Government Agencies
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent	8.40%	15.10%
U.S. Government Agencies | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 9,345	$ 15,166
U.S. Government Agencies | Quoted Prices in Active Markets for Identical Asset (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	9,345	15,166
U.S. Government Agencies | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	0	0
U.S. Government Agencies | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0
Accrued Income
Defined Benefit Plan Disclosure [Line Items]
Asset allocation percent	0.10%	0.10%
Accrued Income | Fair Value, Inputs, Level 1, 2 and 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 174	$ 168
Accrued Income | Quoted Prices in Active Markets for Identical Asset (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	174	168
Accrued Income | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	0	0
Accrued Income | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan fair value of assets	$ 0	$ 0