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Stock Compensation Plans: Stock Option Activity (Details) - $ / shares
 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Number of Shares      
Options outstanding, beginning of period (in shares) 373,130 341,300 221,400
Options granted (in shares) 58,250 55,750 128,000
Options exercised (in shares) (46,310) (21,020) (6,300)
Options forfeited (in shares) (4,950) (2,900) (1,800)
Options outstanding, end of period (in shares) 380,120 373,130 341,300
Weighted Average Exercise Price      
Options outstanding, beginning of period (in dollars per share) $ 9.82 $ 8.73 $ 7.49
Options granted (in dollars per share) 29.69 15.67 10.62
Options exercised (in dollars per share) 7.17 7.56 4.84
Options forfeited (in dollars per share) 6.28 9.96 4.55
Options outstanding, end of period (in dollars per share) $ 13.23 $ 9.82 $ 8.73