Associate (Employee) Benefit Plans - Additional Information (Detail)		12 Months Ended
	Mar. 31, 2014	Dec. 31, 2017 USD ($) plan shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to the plan on behalf of associates, cash expenditure		$ 3,600,000	$ 3,100,000	$ 2,600,000
Percentage of contributions to be invested in balanced fund if no designation made		100.00%
Employee benefit plan, sales of common stock (in shares) | shares		156,000	36,000	25,000
Employee benefit plan purchase of common stock (in shares) | shares		83,000	0	0
Requisite service period	10 years
Amortization of unrecognized gains losses exceed percentage		10.00%
Amortization of net actuarial gain (less than)		$ 100,000
Amortization of net transition obligation		$ 100,000
Percentage of annual medical premium cap		4.00%
Amount of annual health premium per retiree		$ 3,416
Estimated contribution for health fund		3,553
Benefit obligation		1,990,000	$ 1,764,000	$ 1,805,000	$ 2,266,000
Defined benefit plan fair value of assets		0	0	0	$ 0
Net amount recognized		$ 70,000	(505,000)	20,000
Number of additional supplemental plans | plan		5
Maximum
Defined Benefit Plan Disclosure [Line Items]
Average annual rate of increase for medical benefits (less than)		10.00%
Alliance
Defined Benefit Plan Disclosure [Line Items]
Amortization from accumulated other comprehensive income		$ 0
Alliance | Pennsylvania
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation		7,853,000	7,517,000	7,148,000
Defined benefit plan fair value of assets		8,378,000	7,504,000	$ 7,397,000
Net amount recognized		170,000	157,000
Supplemental Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)		700,000	800,000
Early Retirement
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)		100,000	200,000
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)		1,500,000	1,800,000
Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan, corresponding liability (less than)		$ 100,000	$ 100,000