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Note 6 - Stock-based Compensation - Stock Option and Non-vested Stock Award Activity (Details)
 3 Months Ended
Jun. 30, 2018
$ / shares
shares
Options outstanding (in shares) 458,000
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares $ 86.38
Non-vested stock awards outstanding (in shares) 9,000
Non-vested stock awards outstanding, weighted average grant date fair value (in dollars per share) | $ / shares $ 125.68
Options granted (in shares) 24,940
Options granted, weighted average exercise price (in dollars per share) | $ / shares $ 143.36
Non-vested stock awards granted (in shares) 27,000
Non-vested stock awards granted, weighted average grant date fair value (in dollars per share) | $ / shares $ 168.79
Options forfeited or expired (in shares) (22,000)
Options forfeited or expired, weighted average exercise price (in dollars per share) | $ / shares $ 94.45
Non-vested stock awards forfeited (in shares)
Non-vested stock awards forfeited, weighted average grant date fair value (in dollars per share) | $ / shares
Options exercised (in shares) (48,000)
Options exercised, weighted average exercise price (in dollars per share) | $ / shares $ 69.21
Non-vested stock awards vested (in shares) (1,000)
Non-vested stock awards vested, weighted average grant date fair value (in dollars per share) | $ / shares $ 122.98
Options outstanding (in shares) 413,000
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares $ 91.40
Non-vested stock awards outstanding (in shares) 35,000
Non-vested stock awards outstanding, weighted average grant date fair value (in dollars per share) | $ / shares $ 159.29
Options exercisable (in shares) 160,000
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares
Non-vested stock awards vested (in shares) 1,000