Note 14 - Earnings Per Share - Reconciliation of Basic and Diluted Earnings Per Share (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 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
Redeemable noncontrolling interest redemption value adjustment										1,253	(17,970)	909
Net income attributable to common shareholders										$ 253,260	$ 220,287	$ 158,717
Weighted average shares, basic (in shares)										61,926,986	61,662,031	62,040,704
Dilutive effect of stock compensation awards (in shares)	[1]									938,460	571,194	602,168
Diluted shares (in shares)										62,865,446	62,233,225	62,642,872
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
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

[1] Excludes approximately 38,900 and 48,200 stock options for the three and nine months ended September 30, 2018, respectively, as the impact of such awards was anti-dilutive. There were no awards with an anti-dilutive impact for the three and nine months ended September 30, 2019.