Associate (Employee) Benefit Plans - Schedule of Net Periodic Benefit Cost Components of Postretirement Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Benefit obligation at beginning of year	$ 1,764	$ 1,805	$ 2,266
Service cost	53	58	59
Interest cost	71	76	89
Actuarial gain	207	(68)	(502)
Benefits paid	(105)	(107)	(107)
Benefit obligation at end of year	1,990	1,764	1,805
Change in plan assets:
Fair value of plan assets at beginning of year	0	0	0
Employer contributions	105	107	107
Benefits paid	(105)	(107)	(107)
Fair value of plan assets at end of year	0	0	0
Funded status:
Unfunded status	(1,990)	(1,764)	(1,805)
Total (income) recognized in other comprehensive income	(1,348)	(1,701)	(1,271)
Net amount recognized	(3,338)	(3,465)	(3,076)
Components of net periodic benefit cost:
Service cost	53	58	59
Interest cost	71	76	89
Amortization of transition obligation	(76)	(76)	(76)
Net (gain) loss recognition	(70)	505	(20)
Net periodic benefit cost	$ (22)	$ 563	$ 52
Assumption used to determine net periodic benefit cost:
Discount rate	4.10%	4.25%	4.00%
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):	3.60%	4.10%	4.25%
Alliance | Pennsylvania
Change in benefit obligation:
Benefit obligation at beginning of year	$ 7,517	$ 7,148
Service cost	40	40
Interest cost	297	301
Actuarial gain	446	442
Benefits paid	(407)	(374)
Benefit obligation at end of year	7,853	7,517	$ 7,148
Change in plan assets:
Fair value of plan assets at beginning of year	7,504	7,397
Actual return on Plan Assets	1,314	518
Benefits paid	(407)	(374)
Administrative Expenses	(33)	(37)
Fair value of plan assets at end of year	8,378	7,504	$ 7,397
Funded status:
Unfunded status	(7,853)	(7,517)
Total (income) recognized in other comprehensive income	8,378	7,504
Net amount recognized	525	(13)
Components of net periodic benefit cost:
Service cost	40	40
Interest cost	297	301
Expected return on plan assets	(548)	(541)
Net (gain) loss recognition	(170)	(157)
Net periodic benefit cost	$ (381)	$ (357)
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):
Discount rate for Net Periodic Benefit Cost	4.00%	4.00%
Expected Return on Plan Assets	7.50%	7.50%
Assumption used to value the Accumulated Postretirement Benefit Obligation (APBO):	3.60%	4.00%