Commitments - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) a	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule Of Commitments And Contingencies [Line Items]
Rent Expense \| $	$ 1,102	$ 946	$ 947
Hannibal, Missouri
Schedule Of Commitments And Contingencies [Line Items]
Long Term Ground Leases Expiration Period	20 years
Marsing
Schedule Of Commitments And Contingencies [Line Items]
Long Term Ground Leases Expiration Period	25 years
Number of acres owns \| a	15
Number of acres holds on long-term ground lease \| a	2
Vehicle Lease
Schedule Of Commitments And Contingencies [Line Items]
Rent Expense \| $	$ 529	$ 555	$ 435
Minimum \| Axis, Alabama
Schedule Of Commitments And Contingencies [Line Items]
Long Term Ground Leases Expiration Period	20 years
Maximum \| Axis, Alabama
Schedule Of Commitments And Contingencies [Line Items]
Long Term Ground Leases Expiration Period	35 years