Associate Benefit Plans - Schedule of Net Periodic Benefit Cost Components of Postretirement Benefits (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in benefit obligation:
Benefit obligation at beginning of year	$ 2,170	$ 2,170	$ 1,893	$ 1,990
Service cost		61	54	60
Interest cost		67	78	70
Actuarial gain (loss)		80	229	(143)
Benefits paid		(90)	(84)	(84)
Benefit obligation at end of year		2,288	2,170	1,893
Change in plan assets:
Fair value of plan assets at beginning of year	0	0	0	0
Employer contributions		90	84	84
Benefits paid		(90)	(84)	(84)
Fair value of plan assets at end of year		0	0	0
Unfunded status		(2,288)	(2,170)	(1,893)
Amounts recognized in accumulated other comprehensive income:
Net prior service credit		435	511	587
Net (loss) gain		376	491	783
Net amount recognized		811	1,002	1,370
Components of net periodic benefit cost:
Service cost		61	54	60
Interest cost		67	78	70
Amortization of prior service cost		(76)	(76)	(76)
Net gain recognition		(36)	(63)	(45)
Net periodic benefit cost		$ 16	$ (7)	$ 9
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):
Discount rate for Net Periodic Benefit Cost (as percent)		3.20%	4.20%	3.60%
Discount rate for Disclosure Obligations (as percent)		2.40%	3.10%	4.20%
Pension Benefits
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):
Discount rate for Net Periodic Benefit Cost (as percent)		2.91%	4.14%
Expected Return on Plan Assets (as percent)		6.50%	6.50%
Discount rate for Disclosure Obligations (as percent)		3.25%	3.25%
Other Postretirement Benefits | FMS
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):
Discount rate for Net Periodic Benefit Cost (as percent)		2.61%	3.60%
Discount rate for Disclosure Obligations (as percent)		1.49%	2.60%
Other Postretirement Benefits | Split-Dollar
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):
Discount rate for Net Periodic Benefit Cost (as percent)		2.57%	3.54%
Discount rate for Disclosure Obligations (as percent)		1.45%	2.55%
Alliance | Pension Benefits
Components of net periodic benefit cost:
Settlements	1,431
Beneficial | Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	102,878	$ 102,878	$ 0
Acquired in business combinations		0	91,568
Service cost		0	0
Interest cost		2,910	2,851
Plan participants' contributions		0	0
Amendments		0	0
Actuarial gain (loss)		11,499	12,020
Benefits paid		(5,004)	(3,561)
Benefit obligation at end of year		112,283	102,878	$ 0
Change in plan assets:
Fair value of plan assets at beginning of year	100,388	100,388	0
Acquired in business combinations		0	90,814
Actual return on plan assets		15,985	13,350
Employer contributions		340	293
Participants' contributions		0	0
Benefits paid		(5,004)	(3,561)
Administrative expenses		(580)	(508)
Fair value of plan assets at end of year		111,129	100,388	0
Unfunded status		(1,154)	(2,490)
Amounts recognized in accumulated other comprehensive income:
Net (loss) gain		6,635	(4,174)
Components of net periodic benefit cost:
Service cost		0	0
Interest cost		2,910	2,851
Expected return on plan assets		(6,363)	(4,829)
Net gain recognition		4	0
Net periodic benefit cost		(3,449)	(1,978)
Beneficial | Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	20,263	20,263	0
Acquired in business combinations		0	19,504
Service cost		48	76
Interest cost		475	588
Plan participants' contributions		35	34
Amendments		(202)	0
Actuarial gain (loss)		(520)	1,035
Benefits paid		(797)	(974)
Benefit obligation at end of year		19,302	20,263	0
Change in plan assets:
Fair value of plan assets at beginning of year	0	0	0
Acquired in business combinations		0	0
Actual return on plan assets		0	0
Employer contributions		762	940
Participants' contributions		35	34
Benefits paid		(797)	(974)
Administrative expenses		0	0
Fair value of plan assets at end of year		0	0	0
Unfunded status		(19,302)	(20,263)
Amounts recognized in accumulated other comprehensive income:
Net (loss) gain		517	(1,035)
Components of net periodic benefit cost:
Service cost		48	76
Interest cost		475	588
Expected return on plan assets		0	0
Net gain recognition		(205)	0
Net periodic benefit cost		$ 318	$ 664
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):
Discount rate for Net Periodic Benefit Cost (as percent)		3.12%	4.10%
Discount rate for Disclosure Obligations (as percent)		2.31%	3.19%
Pennsylvania | Alliance
Change in benefit obligation:
Benefit obligation at beginning of year	6,893	$ 6,893	$ 6,289	7,853
Service cost	17		40	40
Interest cost	105		253	279
Settlements	(7,272)		(282)	(1,142)
Actuarial gain (loss)	274		864	(430)
Benefits paid	0		(231)	(271)
Benefit obligation at end of year	0		6,893	6,289
Change in plan assets:
Fair value of plan assets at beginning of year	7,431	$ 7,431	6,533	8,378
Actual return on plan assets	(159)		1,435	(393)
Settlements	(7,272)		(273)	(1,146)
Benefits paid	0		(231)	(271)
Administrative expenses	0		(33)	(35)
Fair value of plan assets at end of year	0		7,431	6,533
Unfunded status	0		538	244
Amounts recognized in accumulated other comprehensive income:
Net (loss) gain	0		(279)	(413)
Components of net periodic benefit cost:
Service cost	17		40	40
Interest cost	105		253	279
Expected return on plan assets	(196)		(471)	(596)
Settlements			16	(24)
Net gain recognition	0		0	413
Net periodic benefit cost	$ 1,357		$ (162)	$ 112