Note 17 - Quarterly Financial Information (Unaudited) - Unaudited Quarterly Financial Information (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
Net sales	$ 563,404	$ 562,388	$ 497,581	$ 400,091	$ 493,047	$ 457,747	$ 396,523	$ 332,056	$ 2,023,464	$ 1,679,373	$ 1,447,743
Gross profit	204,306	200,334	178,473	141,927	182,928	157,680	134,224	109,954	725,040	584,786	512,421
Operating income	108,848	106,519	85,467	56,347	95,374	72,548	51,822	30,890	357,181	250,634	202,752
Net income attributable to Generac Holdings Inc.	$ 75,575	$ 75,776	$ 53,261	$ 33,645	$ 80,907	$ 39,435	$ 25,291	$ 12,175	$ 238,257	$ 157,808	$ 97,154
Net income attributable to common shareholders per common share - basic: (in dollars per share)	$ 1.21	$ 1.12	$ 0.83	$ 0.42	$ 1.31	$ 0.64	$ 0.41	$ 0.21	$ 3.57	$ 2.56	$ 1.48
Net income attributable to common shareholders per common share - diluted: (in dollars per share)	$ 1.20	$ 1.11	$ 0.82	$ 0.42	$ 1.30	$ 0.63	$ 0.41	$ 0.20	$ 3.54	$ 2.53	$ 1.47