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Stockholders' Equity (Details 6) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
May 31, 2014
May 31, 2013
May 31, 2012
Summary of stock-based compensation in accordance with authoritative guidance on share based payment awards      
Stock based compensation expense $ 5,502us-gaap_AllocatedShareBasedCompensationExpense $ 4,609us-gaap_AllocatedShareBasedCompensationExpense $ 4,052us-gaap_AllocatedShareBasedCompensationExpense
Tax benefit 1,862us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense 1,540us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense 1,447us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
Stock based compensation expense, net of tax 3,640us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax 3,069us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax 2,605us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax
Cost of Sales [Member]      
Summary of stock-based compensation in accordance with authoritative guidance on share based payment awards      
Stock based compensation expense 232us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
271us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
268us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_CostOfSalesMember
Research and Development [Member]      
Summary of stock-based compensation in accordance with authoritative guidance on share based payment awards      
Stock based compensation expense 483us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember
399us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember
738us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_ResearchAndDevelopmentExpenseMember
Sales and Marketing [Member]      
Summary of stock-based compensation in accordance with authoritative guidance on share based payment awards      
Stock based compensation expense 1,672us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SellingAndMarketingExpenseMember
1,610us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SellingAndMarketingExpenseMember
1,340us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SellingAndMarketingExpenseMember
General and Administrative [Member]      
Summary of stock-based compensation in accordance with authoritative guidance on share based payment awards      
Stock based compensation expense 3,115us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember
2,329us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember
1,706us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_GeneralAndAdministrativeExpenseMember
Operating Expenses [Member]      
Summary of stock-based compensation in accordance with authoritative guidance on share based payment awards      
Stock based compensation expense $ 5,270us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OperatingExpenseMember
$ 4,338us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OperatingExpenseMember
$ 3,784us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OperatingExpenseMember