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10. BORROWING ARRANGEMENTS - Borrowings (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Borrowing Arrangements [Line Items]    
Short-term portion of borrowings, Amount $ 3,500us-gaap_ShortTermBorrowings $ 8,000us-gaap_ShortTermBorrowings
Long-term borrowings, Amount 7,500us-gaap_LongTermDebt 8,000us-gaap_LongTermDebt
Total of Short-term and Long-term borrowings, Amount 11,000us-gaap_DebtLongtermAndShorttermCombinedAmount  
Short And Long Term Borrowings [Member]    
Borrowing Arrangements [Line Items]    
Short-term portion of borrowings, Amount 3,500us-gaap_ShortTermBorrowings
/ us-gaap_DebtInstrumentAxis
= amrb_ShortAndLongTermBorrowingsMember
8,000us-gaap_ShortTermBorrowings
/ us-gaap_DebtInstrumentAxis
= amrb_ShortAndLongTermBorrowingsMember
Long-term borrowings, Amount 7,500us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= amrb_ShortAndLongTermBorrowingsMember
8,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= amrb_ShortAndLongTermBorrowingsMember
Total of Short-term and Long-term borrowings, Amount $ 11,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= amrb_ShortAndLongTermBorrowingsMember
$ 16,000us-gaap_DebtLongtermAndShorttermCombinedAmount
/ us-gaap_DebtInstrumentAxis
= amrb_ShortAndLongTermBorrowingsMember
Weighted Average Interest Rate [Member]    
Borrowing Arrangements [Line Items]    
Short-term portion of borrowings, Weighted Average Rate 0.92%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= amrb_WeightedAverageInterestRateMember
2.15%us-gaap_ShortTermDebtWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= amrb_WeightedAverageInterestRateMember
Long-term borrowings, Weighted Average Rate 1.39%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= amrb_WeightedAverageInterestRateMember
1.47%us-gaap_LongtermDebtWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= amrb_WeightedAverageInterestRateMember
Total of Short-term and Long-term borrowings, Weighted Average Rate 1.24%us-gaap_DebtWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= amrb_WeightedAverageInterestRateMember
1.81%us-gaap_DebtWeightedAverageInterestRate
/ us-gaap_DebtInstrumentAxis
= amrb_WeightedAverageInterestRateMember