Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues as originally reported	$ 150,330	$ 147,769	$ 145,587	$ 141,569	$ 141,261	$ 134,558	$ 130,998	$ 126,999	$ 584,933	$ 533,647	$ 482,916
Net earnings	72,185	61,129	58,409	73,648	66,126	50,772	51,933	70,676	265,371	239,506	197,961
Net earnings attributable to NNN	$ 72,168	$ 61,120	$ 58,028	$ 73,657	$ 66,092	$ 50,784	$ 51,942	$ 70,683	$ 264,973	$ 239,500	$ 197,836
Net earnings per share:
Basic (in dollars per share)	$ 0.42	$ 0.35	$ 0.33	$ 0.35	$ 0.37	$ 0.29	$ 0.30	$ 0.44	$ 1.45	$ 1.39	$ 1.21
Diluted (in dollars per share)	$ 0.42	$ 0.35	$ 0.33	$ 0.35	$ 0.37	$ 0.28	$ 0.30	$ 0.44	$ 1.45	$ 1.38	$ 1.20