Note 10 - Earnings Per Share - Summary of Earnings and Weighted Average Number of Common Shares Used in Calculation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Basic:
Weighted average common shares outstanding (in shares)	15,364,043	15,310,754	15,347,042	15,304,235
Net income (loss) attributable to National HealthCare Corporation	$ (3,348)	$ 12,849	$ 122,802	$ 14,321
Earnings (loss) per common share, basic (in dollars per share)	$ (0.22)	$ 0.84	$ 8.00	$ 0.94
Diluted:
Weighted average common shares outstanding (in shares)	15,364,043	15,310,754	15,347,042	15,304,235
Effects of dilutive instruments (in shares)	0	60,557	67,641	64,540
Weighted average common shares outstanding (in shares)	15,364,043	15,371,311	15,414,683	15,368,775
Net income (loss) attributable to National HealthCare Corporation	$ (3,348)	$ 12,849	$ 122,802	$ 14,321
Earnings (loss) per common share, diluted (in dollars per share)	$ (0.22)	$ 0.84	$ 7.97	$ 0.93
Share-based Payment Arrangement, Option [Member]
Basic:
Weighted average common shares outstanding (in shares)	15,364,043	15,310,754	15,347,042	15,304,235
Diluted:
Weighted average common shares outstanding (in shares)	15,364,043	15,310,754	15,347,042	15,304,235