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Commitments - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Schedule Of Commitments And Contingencies [Line Items]      
Rent Expense $ 1,012us-gaap_LeaseAndRentalExpense $ 939us-gaap_LeaseAndRentalExpense $ 727us-gaap_LeaseAndRentalExpense
Hannibal, Missouri      
Schedule Of Commitments And Contingencies [Line Items]      
Long Term Ground Leases Expiration Period   20 years 20 years
Marsing      
Schedule Of Commitments And Contingencies [Line Items]      
Long Term Ground Leases Expiration Period   25 years 25 years
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