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Basis of Presentation - Additional Information (Detail)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]    
Percentage of comparable consolidated amounts 95.00%amsf_PercentageOfComparableConsolidatedAmount 95.00%amsf_PercentageOfComparableConsolidatedAmount