Recent Accounting Pronouncements (Details)	6 Months Ended
Jun. 30, 2012
Recent Accounting Pronouncements [Abstract]
Minimum percentage of likelihood for more-likely-than-not threshold (in hundredths)	50.00%

Refers to the percentage of likelihood for more likely than not threshold.

No definition available.

Name:	amnb_MinimumPercentageOfLikelihoodForMoreLikelyThanNotThreshold
Namespace Prefix:	amnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration