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Organization and Summary of Significant Accounting Policies (Summary of NNN's Investment Portfolio) (Details) ft² in Thousands | 12 Months Ended
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Dec. 31, 2018 ft² state property
Property Portfolio: |
Total properties | property | 2,969
Gross leasable area (square feet) | ft² | 30,487
States | state | 48
Weighted average remaining lease term (years) | 11 years 6 months

X
- Definition Investment Properties Total + References No definition available. + Details
Name: | nnn_InvestmentPropertiesTotal
Namespace Prefix: | nnn_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Weighted Average Remaining Lease Term + References No definition available. + Details
Name: | nnn_WeightedAverageRemainingLeaseTerm
Namespace Prefix: | nnn_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Area of a real estate property. + References No definition available. + Details
Name: | us-gaap_AreaOfRealEstateProperty
Namespace Prefix: | us-gaap_
Data Type: | num:areaItemType
Balance Type: | na
Period Type: | instant

X
- Definition The number of states the entity operates in as of the balance sheet date. + References No definition available. + Details
Name: | us-gaap_NumberOfStatesInWhichEntityOperates
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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