Associate Benefit Plans - Schedule of Net Periodic Benefit Cost Components of Postretirement Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in benefit obligation:
Benefit obligation at beginning of year	$ 1,990	$ 1,764	$ 1,805
Service cost	60	53	58
Interest cost	70	71	76
Actuarial gain	(143)	207	(68)
Benefits paid	(84)	(105)	(107)
Benefit obligation at end of year	1,893	1,990	1,764
Change in plan assets:
Fair value of plan assets at beginning of year	0	0	0
Employer contributions	84	105	107
Benefits paid	(84)	(105)	(107)
Funded status:
Unfunded status	1,990	1,764	1,805	$ 1,893	$ 1,990	$ 1,764
Total loss (income) recognized in other comprehensive income	0	0	0	0	0	0
Net amount recognized				$ (1,893)	$ (1,990)	$ (1,764)
Components of net periodic benefit cost:
Service cost	60	53	58
Interest cost	70	71	76
Amortization of transition obligation	(76)	(76)	(76)
Net (gain) loss recognition	(45)	(70)	505
Net periodic benefit cost	$ 9	$ (22)	$ 563
Assumption used to determine net periodic benefit cost:
Discount rate (as percent)	3.60%	4.10%	4.25%
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):
Discount rate for Disclosure Obligations (as percent)				4.20%	3.60%	4.10%
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	$ (1,370)	$ (1,348)	$ (1,701)
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax	(3,263)	(3,338)	(3,465)
Pennsylvania | Alliance
Change in benefit obligation:
Benefit obligation at beginning of year	7,853	7,517	7,148
Service cost	40	40	40
Interest cost	279	297	301
Settlements	(1,142)	0	0
Actuarial gain	(430)	446	442
Benefits paid	(271)	(407)	(374)
Benefit obligation at end of year	6,289	7,853	7,517
Change in plan assets:
Fair value of plan assets at beginning of year	8,378	7,504	7,397
Actual return on Plan Assets	(393)	1,314	518
Settlements	(1,146)	0	0
Benefits paid	(271)	(407)	(374)
Administrative Expenses	(35)	(33)	(37)
Funded status:
Unfunded status	7,853	7,517	7,148	$ 6,289	$ 7,853	$ 7,517
Total loss (income) recognized in other comprehensive income	8,378	7,504	7,397	6,533	8,378	7,504
Net amount recognized				$ (6,289)	$ (7,853)	$ (7,517)
Components of net periodic benefit cost:
Service cost	40	40	40
Interest cost	279	297	301
Expected return on plan assets	(596)	(548)	(541)
Settlements	(24)	0	0
Net (gain) loss recognition	413	(170)	(157)
Net periodic benefit cost	$ 112	$ (381)	$ (357)
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):
Discount rate for Net Periodic Benefit Cost (as percent)	3.60%	4.00%	4.00%
Expected Return on Plan Assets (as percent)	7.50%	7.50%	7.50%
Discount rate for Disclosure Obligations (as percent)				4.20%	3.60%	4.00%
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	$ 6,533	$ 8,378	$ 7,504
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax	$ 244	$ 525	$ (13)