Capital Requirements - Narrative (Detail)	3 Months Ended
Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018	Jan. 01, 2017	Jan. 01, 2016
Regulatory Capital Requirements [Abstract]
Capital conservation buffer period	4 years
Capital conservation buffer percentage	2.50%	0.625%	1.875%	0.625%	0.625%	0.625%
Capital conservation buffer percentage maintained at bank	6.49%		5.62%

Capital conservation buffer percentage maintained at bank.

No definition available.

Name:	fxnc_CapitalConservationBufferPercentageMaintainedAtBank
Namespace Prefix:	fxnc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant