Real Estate (Real Estate Held for Sale) (Details) $ in Thousands	Mar. 31, 2019 USD ($) property	Dec. 31, 2018 USD ($)
Real Estate [Abstract]
Number of properties classified as held for sale \| property	7
Land and improvements	$ 13,982	$ 13,976
Building and improvements	34,166	34,166
Real estate held-for-sale	48,148	48,142
Less accumulated depreciation and amortization	(10,608)	(10,547)
Less impairment	(16,143)	(14,250)
Real estate held for sale	$ 21,397	$ 23,345

Accumulated Impairment Real Estate Held For Sale

No definition available.

Name:	nnn_AccumulatedImpairmentRealEstateHeldForSale
Namespace Prefix:	nnn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Inventory, Building, Held-for-sale

+ References

No definition available.

+ Details

Name:	nnn_InventoryBuildingHeldForSale
Namespace Prefix:	nnn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Inventory, Real Estate, Held-For-Sale, Accumulated Depreciation

+ References

No definition available.

+ Details

Name:	nnn_InventoryRealEstateHeldForSaleAccumulatedDepreciation
Namespace Prefix:	nnn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number Of Properties Classified As Held For Sale

+ References

No definition available.

+ Details

Name:	nnn_NumberOfPropertiesClassifiedAsHeldForSale
Namespace Prefix:	nnn_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of parcels of land held for sale, and on which the entity has no intention or no further intention to perform construction activities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.6(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Represents properties which were acquired directly or through foreclosure for which a committed plan to sell exists and an active program to market such properties has been initiated.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Carrying amount as of the balance sheet date of investments in land and buildings held for sale, excluding real estate considered to be inventory of the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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