Employee Benefit Plans - Changes in Accumulated Benefit Obligation and Fair Value of Qualified Trust Assets (Pension Plan only) and Statement of Funded (Unfunded) Status for Pension Plan and PRB Plan (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Pension Plan
Change in benefit obligation
Benefit obligation, beginning of year	$ 3,273,000	$ 3,398,000
Interest cost	94,000	121,000
Actuarial (gain) loss	(694,000)	323,000
Benefit payments	(1,295,000)	(581,000)
Settlement loss	221,000	12,000
Benefit obligation, end of year	1,599,000	3,273,000
Change in plan assets
Fair value of plan assets, beginning of year	2,465,000	2,690,000
Actual (loss) return on plan assets	(54,000)	356,000
Employer contributions	0
Benefits payments	(1,295,000)	(581,000)
Fair value of plan assets, end of year	1,116,000	2,465,000
(Unfunded) funded status, end of year	(483,000)	(808,000)
PRB Plan
Change in benefit obligation
Benefit obligation, beginning of year	457,000	540,000
Service cost	3,000	22,000
Interest cost	5,000	21,000
Actuarial (gain) loss	(36,000)	(120,000)
Benefit payments	(6,000)	(6,000)
Curtailment/termination (gain)	(352,000)
Benefit obligation, end of year	71,000	457,000
Change in plan assets
Employer contributions	6,000	6,000
Benefits payments	(6,000)	(6,000)
(Unfunded) funded status, end of year	$ (71,000)	$ (457,000)