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Note 14 - Employee Benefits (Details Textual) - USD ($)		1 Months Ended											12 Months Ended
Note 14 - Employee Benefits (Details Textual) - USD ($)	May 07, 2019	May 31, 2019	Dec. 31, 2018	Aug. 31, 2018	May 31, 2018	Aug. 31, 2017	May 31, 2017	Aug. 31, 2016	May 31, 2016	May 31, 2015	May 31, 2014	May 31, 2013	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted													1,133,362
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													$ 1,817,000	$ 3,323,000	$ 1,250,000
Share-based Payment Arrangement, Expense, after Tax													$ 1,400,000	$ 2,600,000	$ 738,000
Allocated Share Based Compensation Expense Net Of Tax Amount Per Basic And Diluted Share													$ 0.03	$ 0.05	$ 0.01
Income Tax Expense (Benefit), Total													$ 16,617,000	$ 16,002,000	$ (33,811,000)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance			939,193										842,329	939,193
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term													3 years 4 months 24 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value													$ 5,800,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number													523,677
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term													2 years 4 months 24 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value													$ 4,600,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value													1,400,000	$ 1,500,000	830,000
Proceeds from Stock Options Exercised													1,600,000	938,000	1,100,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value													$ 569,000	547,000	515,000
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													2,700,000	2,300,000	2,100,000
Retirement Savings Plan, Participants Less Than 50 Years Old [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Amount													19,000
Retirement Savings Plan, Participants Over50 Years Old [Member]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Amount													25,000
Accounting Standards Update 2016-09 [Member]
Income Tax Expense (Benefit), Total													$ 341,000	319,000	176,000
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period												5 years	5 years
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													$ 1,200,000
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		40,000			45,700		109,169		57,669	58,335	60,668	104,250
Share-based Compensation Arrangement By Share-based Payment Awards, Annual Service Vesting Percentage							10.00%
Performance Shares [Member] \| Awards Paid to Employees Upon Vesting [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted		30,000	2,000	2,000	28,000	2,000	43,342	1,667	21,671	32,500	18,337
Unvested Service Based Option Awards [Member]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total													$ 1,400,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition													3 years 1 month 6 days
Unvested Performance Unit Awards [Member]
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total													$ 2,400,000
Service Based Options [Member]
Shares Granted, Value, Share-based Payment Arrangement, before Forfeiture													$ 401,000	$ 957,000	$ 592,000
2015 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	1,300,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized													2,633,333
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%
Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Reserved For Issuance Under Awards Outstanding													587,672
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													247,621
2015 Equity Incentive Plan [Member] \| Full Value Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	558,334
The 2005 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant			0											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													254,657
The 2005 Stock Incentive Plan [Member] \| Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Reserved For Issuance Under Awards Outstanding													0
Minimum [Member] \| 2015 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period													7 years
Maximum [Member] \| 2015 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period													10 years