Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues as originally reported	$ 160,529	$ 155,526	$ 155,555	$ 152,861	$ 150,330	$ 147,769	$ 145,587	$ 141,569	$ 622,661	$ 584,933	$ 533,647
Net earnings	36,572	82,042	70,583	103,289	72,185	61,129	58,409	73,648	292,485	265,371	239,506
Net earnings attributable to NNN	$ 36,562	$ 82,032	$ 70,573	$ 103,280	$ 72,168	$ 61,120	$ 58,028	$ 73,657	$ 292,447	$ 264,973	$ 239,500
Net earnings per share:
Basic (in dollars per share)	$ 0.17	$ 0.47	$ 0.40	$ 0.62	$ 0.42	$ 0.35	$ 0.33	$ 0.35	$ 1.65	$ 1.45	$ 1.39
Diluted (in dollars per share)	$ 0.17	$ 0.47	$ 0.40	$ 0.62	$ 0.42	$ 0.35	$ 0.33	$ 0.35	$ 1.65	$ 1.45	$ 1.38