Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues as originally reported	$ 173,576	$ 169,108	$ 165,279	$ 165,636	$ 160,529	$ 155,526	$ 155,555	$ 152,861	$ 670,487	$ 622,661	$ 584,933
Net earnings	73,786	66,698	79,091	80,033	36,572	82,042	70,583	103,289	299,608	292,485	265,371
Net earnings attributable to NNN	$ 73,786	$ 66,693	$ 78,678	$ 80,023	$ 36,562	$ 82,032	$ 70,573	$ 103,280	$ 299,180	$ 292,447	$ 264,973
Net earnings per share:
Basic (in dollars per share)	$ 0.34	$ 0.35	$ 0.43	$ 0.44	$ 0.17	$ 0.47	$ 0.40	$ 0.62	$ 1.56	$ 1.65	$ 1.45
Diluted (in dollars per share)	$ 0.34	$ 0.35	$ 0.43	$ 0.44	$ 0.17	$ 0.47	$ 0.40	$ 0.62	$ 1.56	$ 1.65	$ 1.45