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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Research and Development (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Significant Accounting Policies [Line Items]      
Research and development expenses to sales, percent 3.00%ntks_ResearchAndDevelopmentExpensesToSalesRevenueNetPercentage 2.90%ntks_ResearchAndDevelopmentExpensesToSalesRevenueNetPercentage 2.70%ntks_ResearchAndDevelopmentExpensesToSalesRevenueNetPercentage
SG&A [Member]      
Significant Accounting Policies [Line Items]      
Research and development expense 75.4us-gaap_ResearchAndDevelopmentExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SellingGeneralAndAdministrativeExpensesMember
65.5us-gaap_ResearchAndDevelopmentExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SellingGeneralAndAdministrativeExpensesMember
59.4us-gaap_ResearchAndDevelopmentExpense
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_SellingGeneralAndAdministrativeExpensesMember