Loans Receivable and Allowance for Loan Losses - Loans Receivable Delinquencies (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Loans in repayment status:
Total loans	$ 21,206,948	$ 22,520,498
Federally insured loans
Loans in repayment status:
Total loans	20,695,837	22,155,896
Federally insured loans \| Federally insured loans
Financing Receivable, Recorded Investment [Line Items]
Loans in-school/grace/deferment	1,243,705	1,298,493	$ 1,410,902
Loans in forbearance	1,391,482	1,430,291	1,487,107
Loans in repayment status:
Loans current	15,646,231	16,882,252	16,921,119
Total loans in repayment	$ 18,060,650	$ 19,427,112	$ 19,492,734
Loans current, percentage	86.70%	86.90%	86.80%
Total loans in repayment, percentage	100.00%	100.00%	100.00%
Total loans	$ 20,695,837	$ 22,155,896	$ 22,390,743
Federally insured loans \| Loans delinquent 31-60 days \| Federally insured loans
Loans in repayment status:
Loans past due	$ 662,431	$ 683,084	$ 689,454
Loans past due, percentage	3.80%	3.50%	3.50%
Federally insured loans \| Loans delinquent 61-90 days \| Federally insured loans
Loans in repayment status:
Loans past due	$ 402,197	$ 427,764	$ 412,639
Loans past due, percentage	2.20%	2.20%	2.10%
Federally insured loans \| Loans delinquent 91-120 days \| Federally insured loans
Loans in repayment status:
Loans past due	$ 279,524	$ 283,831	$ 347,013
Loans past due, percentage	1.50%	1.50%	1.80%
Federally insured loans \| Loans delinquent 121-270 days \| Federally insured loans
Loans in repayment status:
Loans past due	$ 795,230	$ 806,692	$ 853,224
Loans past due, percentage	4.40%	4.20%	4.40%
Federally insured loans \| Loans delinquent 271 days or greater \| Federally insured loans
Loans in repayment status:
Loans past due	$ 275,037	$ 343,489	$ 269,285
Loans past due, percentage	1.40%	1.70%	1.40%
Private education loans
Loans in repayment status:
Total loans	$ 189,912	$ 225,975
Private education loans \| Private education loans
Financing Receivable, Recorded Investment [Line Items]
Loans in-school/grace/deferment	3,944	4,320	$ 3,550
Loans in forbearance	2,242	1,494	1,577
Loans in repayment status:
Loans current	173,883	208,977	156,383
Total loans in repayment	$ 183,726	$ 220,161	$ 164,340
Loans current, percentage	94.70%	95.00%	95.20%
Total loans in repayment, percentage	100.00%	100.00%	100.00%
Total loans	$ 189,912	$ 225,975	$ 169,467
Private education loans \| Loans delinquent 31-60 days \| Private education loans
Loans in repayment status:
Loans past due	$ 3,011	$ 3,626	$ 1,796
Loans past due, percentage	1.60%	1.60%	1.10%
Private education loans \| Loans delinquent 61-90 days \| Private education loans
Loans in repayment status:
Loans past due	$ 1,370	$ 1,560	$ 1,155
Loans past due, percentage	0.70%	0.70%	0.70%
Private education loans \| Loans delinquent 91 days or greater \| Private education loans
Loans in repayment status:
Loans past due	$ 5,462	$ 5,998	$ 5,006
Loans past due, percentage	3.00%	2.70%	3.00%
Consumer loans
Loans in repayment status:
Total loans	$ 321,199	$ 138,627
Consumer loans \| Consumer loans
Loans in repayment status:
Loans current	315,708	136,130	$ 110,885
Total loans in repayment	$ 321,199	$ 138,627	$ 112,547
Loans current, percentage	98.30%	98.20%	98.50%
Total loans in repayment, percentage	100.00%	100.00%	100.00%
Total loans	$ 321,199	$ 138,627	$ 112,547
Consumer loans \| Loans delinquent 31-60 days \| Consumer loans
Loans in repayment status:
Loans past due	$ 2,249	$ 1,012	$ 905
Loans past due, percentage	0.70%	0.70%	0.80%
Consumer loans \| Loans delinquent 61-90 days \| Consumer loans
Loans in repayment status:
Loans past due	$ 1,617	$ 832	$ 355
Loans past due, percentage	0.50%	0.60%	0.30%
Consumer loans \| Loans delinquent 91 days or greater \| Consumer loans
Loans in repayment status:
Loans past due	$ 1,625	$ 653	$ 402
Loans past due, percentage	0.50%	0.50%	0.40%

Loans in Forbearance

No definition available.

Name:	nni_LoansInForbearance
Namespace Prefix:	nni_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Loans in Repayment, Current, in Percentage

+ References

No definition available.

+ Details

Name:	nni_LoansInRepaymentCurrentInPercentage
Namespace Prefix:	nni_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Loans in Repayment Status [Abstract]

+ References

No definition available.

+ Details

Name:	nni_LoansInRepaymentStatusAbstract
Namespace Prefix:	nni_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Loans in School Grace and Deferment

+ References

No definition available.

+ Details

Name:	nni_LoansInSchoolGraceAndDeferment
Namespace Prefix:	nni_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Total Loans in Repayment

+ References

No definition available.

+ Details

Name:	nni_TotalLoansInRepayment
Namespace Prefix:	nni_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Total Loans in Repayment in Percentage

+ References

No definition available.

+ Details

Name:	nni_TotalLoansInRepaymentInPercentage
Namespace Prefix:	nni_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of financing receivable balance that is past due.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivablePercentPastDue1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of financing receivable not past due.

+ References

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of financing receivable past due.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=118955255&loc=SL82922355-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919253-210447

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953401-111524

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentPastDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before allowance for credit loss, of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919230-210447

+ Details

Name:	us-gaap_NotesReceivableGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=nni_ConsumerPortfolioSegmentFederallyInsuredMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=nni_FederallyInsuredLoansExcludingRehabiliationLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=nni_FinancingReceivables31to60DaysPastDueMemberMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=nni_FinancingReceivables61to90DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=nni_Financingreceivables91120dayspastdueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=nni_Financingreceivables121270dayspastdueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=nni_Financingreceivables271daysorgreaterpastdueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=nni_ConsumerPortfolioSegmentPrivateEducationLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=nni_NonFederallyInsuredStudentLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=nni_FinancingReceivablesEqualtoGreaterthan91DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=nni_ConsumerPortfolioSegmentConsumerLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: