Note 20 - Selected Quarterly Financial Data - Selected Quarterly Financial Data (Details) - USD ($)	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
Net operating revenues	$ 253,880,000	$ 247,067,000	$ 247,151,000	$ 248,285,000	$ 248,920,000	$ 246,326,000	$ 242,142,000	$ 242,961,000	$ 996,383,000	$ 980,349,000	$ 963,895,000
Income from operations	16,497,000	6,568,000	10,189,000	15,784,000	17,194,000	10,834,000	12,165,000	15,883,000	49,038,000	56,076,000	54,110,000
Non–operating income	5,811,000	6,663,000	8,272,000	6,001,000	6,614,000	8,467,000	5,654,000	(3,065,000)
Unrealized gains (losses) on marketable equity securities	(3,866,000)	9,312,000	(54,000)	6,838,000	721,000	3,486,000	12,448,000	(15,517,000)	$ 12,230,000	$ 1,138,000	$ 0
Net income attributable to NHC	$ 13,770,000	$ 19,461,000	$ 13,711,000	$ 21,269,000	$ 18,152,000	$ 21,142,000	$ 22,461,000	$ (2,791,000)
Basic earnings per share (in dollars per share)	$ 0.91	$ 1.27	$ 90	$ 1.39	$ 1.19	$ 1.39	$ 1.47	$ (0.18)	$ 4.47	$ 3.87	$ 3.70
Diluted earnings per share (in dollars per share)	$ 0.89	$ 1.27	$ 89	$ 1.39	$ 1.19	$ 1.39	$ 1.47	$ (0.18)	$ 4.44	$ 3.87	$ 3.69