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Major Customer (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Concentration Risk [Line Items]    
Concentration Risk, Customer Department of Education Department of Education
Concentration Risk Dollar Value [Member]    
Concentration Risk [Line Items]    
Loan and guaranty servicing revenue 32.4nni_LoanAndGuarantyServicingRevenue
/ us-gaap_ConcentrationRiskByTypeAxis
= nni_ConcentrationRiskDollarValueMember
29.9nni_LoanAndGuarantyServicingRevenue
/ us-gaap_ConcentrationRiskByTypeAxis
= nni_ConcentrationRiskDollarValueMember