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Employee Benefit Plan (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Retirement Benefits [Abstract] |  |  |
Percentage of participant's contributions matched by employer | 60.00% |  |
Employer matching contribution, percent | 8.00% |  |
Employer contributions | $ 514 | $ 491 | $ 474

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- Definition Defined Contribution Plan, Employer Contribution Amount + References No definition available. + Details
Name: | nnn_DefinedContributionPlanEmployerContributionAmount
Namespace Prefix: | nnn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration