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Note 13 - Employee Benefits (Details Textual) |  |  |  |  | 1 Months Ended | 5 Months Ended | 12 Months Ended
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Aug. 13, 2020 $ / shares shares | May 07, 2019 shares | Jul. 07, 2017 | May 01, 2014 shares | May 31, 2020 shares | May 31, 2019 shares | Dec. 31, 2018 shares | Aug. 31, 2018 shares | May 31, 2018 $ / shares shares | Aug. 31, 2017 shares | May 31, 2017 shares | Aug. 31, 2016 shares | May 31, 2016 shares | May 31, 2015 shares | May 31, 2014 shares | May 31, 2015 shares | Dec. 31, 2020 USD ($) $ / shares shares | Dec. 31, 2019 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) $ / shares
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,815,956 |  |
Common Stock, Par or Stated Value Per Share (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  | $ 0.01 |  |  |  |  |  |  |  | $ 0.01 | $ 0.01 |
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Forfeiture Rate Of Unvested Outstanding Awards |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.25% |  |
Share-based Payment Arrangement, Expense |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,851,000 | $ 1,817,000 | $ 3,323,000
Share-based Payment Arrangement, Expense, after Tax |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,400,000 | $ 1,400,000 | $ 2,600,000
Allocated Share Based Compensation Expense Net Of Tax Amount Per Basic And Diluted Share (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.02 | $ 0.02 | $ 0.03
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 711,780 | 1,263,513 |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 years 9 months 18 days |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 3,900,000 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 379,218 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years 10 months 24 days |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,500,000 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4,100,000 | $ 1,400,000 | $ 1,500,000
Proceeds from Stock Options Exercised |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4,800,000 | 1,600,000 | 938,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 539,000 | 569,000 | 547,000
Defined Contribution Plan, Employer Matching Contribution, Percent of Match |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 35.00% |  |
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% |  |
Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |
Defined Contribution Plan, Employer Discretionary Contribution Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 | 0 | 0
Defined Contribution Plan, Cost |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3,000,000.0 | 2,700,000 | 2,300,000
Retirement Savings Plan, Participants Less Than 50 Years Old [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 19,500 |  |
Retirement Savings Plan, Participants Over50 Years Old [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 26,000 |  |
Stock Split To [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity Note, Stock Split, Conversion Ratio | 3 |  | 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock, Par or Stated Value Per Share (in dollars per share) | $ / shares | $ 0.01 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity Note, Stock Split, Dividend Settlement, Percent | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock Split from [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stockholders' Equity Note, Stock Split, Conversion Ratio | 2 |  | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Option [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Reserved For Issuance Under Awards Outstanding (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 107,048 |  |
Performance Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  | 5 years |  |  |  |  |  |  |  |  |  |  |  |  | 5 years |  |
Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Reserved For Issuance Under Awards Outstanding (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 604,732 |  |
Share-based Compensation Arrangement By Share-based Payment Awards, Annual Service Vesting Percentage |  |  |  | 5.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,300,000 |  |
Performance Shares [Member] | Awards Paid to Employees Upon Vesting [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted (in shares) | shares |  |  |  | 27,511 | 73,205 | 45,000 | 3,000 | 3,000 | 42,000 | 3,000 | 65,013 | 2,501 | 32,513 |  |  | 48,750 |  |  |
Performance Shares [Member] | One Half of Awards Paid Immediately Upon Vesting and One Half Credited to Employees' Accounts [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted (in shares) | shares |  |  |  |  |  | 60,000 |  |  | 68,550 |  | 163,754 |  | 86,505 | 87,503 |  |  |  |  |
Share-based Compensation Arrangement By Share-based Payment Awards, Annual Service Vesting Percentage |  |  |  |  |  |  |  |  |  |  | 10.00% |  |  |  |  |  |  |  |
Unvested Service Based Option Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,100,000 |  |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 years 9 months 18 days |  |
Unvested Performance Unit Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,200,000 |  |
Service Based Options [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares Granted, Value, Share-based Payment Arrangement, before Forfeiture |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 160,000 | $ 401,000 | $ 957,000
2015 Equity Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized (in shares) | shares |  | 1,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2015 Equity Incentive Plan [Member] | Stock Split To [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized (in shares) | shares | 3,950,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
2015 Equity Incentive Plan [Member] | Share-based Payment Arrangement, Option [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award Annual Percentage Of Award Which Vests |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |
2015 Equity Incentive Plan [Member] | Performance Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Reserved For Issuance Under Awards Outstanding (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 318,711 |  |
2015 Equity Incentive Plan [Member] | Full Value Awards [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized (in shares) | shares |  | 558,334 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
The 2005 Stock Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 91,002 |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |
The 2005 Stock Incentive Plan [Member] | Share-based Payment Arrangement, Option [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Reserved For Issuance Under Awards Outstanding (in shares) | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |
Minimum [Member] | 2015 Equity Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 7 years |  |
Maximum [Member] | 2015 Equity Incentive Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period (Year) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10 years |  |

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