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Junior Subordinated Debt - Narrative (Detail) - USD ($)
$ in Millions
Jul. 31, 2006
Jun. 17, 2004
Mar. 31, 2019
Dec. 31, 2018
First National (VA) Statutory Trust II        
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]        
Issuance of trust preferred securities   $ 5.0    
LIBOR-indexed floating rate of interest     5.21% 5.39%
Junior subordinated debt     $ 5.2  
First National (VA) Statutory Trust III        
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]        
Issuance of trust preferred securities $ 4.0      
LIBOR-indexed floating rate of interest     4.40% 4.00%
Junior subordinated debt     $ 4.1  
Maximum capital required for capital adequacy     25.00%