Note 19 - Quarterly Financial Information (Unaudited) - Unaudited Quarterly Financial Information (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
Net sales	$ 590,932	$ 601,135	$ 541,916	$ 470,353	$ 563,404	$ 562,388	$ 497,581	$ 400,091	$ 2,204,336	$ 2,023,464	$ 1,679,373
Gross profit	222,222	217,517	195,838	162,175	204,306	200,334	178,473	141,927	797,752	725,040	584,786
Operating income	104,508	105,556	90,926	71,173	108,848	106,519	85,467	56,347	372,163	357,181	250,634
Net income attributable to Generac Holdings Inc.	$ 69,614	$ 75,574	$ 61,958	$ 44,861	$ 75,575	$ 75,776	$ 53,261	$ 33,645	$ 252,007	$ 238,257	$ 157,808
Net income attributable to common shareholders per common share - basic: (in dollars per share)	$ 1.14	$ 1.20	$ 0.99	$ 0.77	$ 1.21	$ 1.12	$ 0.83	$ 0.42	$ 4.09	$ 3.57	$ 2.56
Net income attributable to common shareholders per common share - diluted: (in dollars per share)	$ 1.12	$ 1.18	$ 0.98	$ 0.76	$ 1.20	$ 1.11	$ 0.82	$ 0.42	$ 4.03	$ 3.54	$ 2.53