Note 1 - Description of Business and Basis of Presentation - Impact of Adopting Accounting Standards (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounts receivable, less allowance for doubtful accounts	$ 341,758	$ 316,419	$ 341,758	$ 316,419	$ 279,294
Inventories	496,088	356,382	496,088	356,382	387,049
Other accrued liabilities	136,416	114,946	136,416	114,946	112,618
Deferred income taxes	68,380	46,161	68,380	46,161	41,852
Other long-term liabilities	93,023	75,734	93,023	75,734	82,893
Retained earnings	756,636	529,929	756,636	529,929	610,835
Accounts receivable	341,758	316,419	341,758	316,419	279,294
Inventories	496,088	356,382	496,088	356,382	387,049
Other accrued liabilities	136,416	114,946	136,416	114,946	112,618
Deferred income taxes	68,380	46,161	68,380	46,161	41,852
Other long-term liabilities	93,023	75,734	93,023	75,734	82,893
Retained earnings	756,636	529,929	756,636	529,929	610,835
Net sales	559,515	455,839	1,452,098	1,181,199
Costs of goods sold	361,630	299,608	937,968	783,247
Selling and service	46,536	43,463	135,270	124,940
Research and development	13,653	10,850	38,122	31,690
General and administrative	25,499	22,128	75,613	64,508
Other, net	483	1,710	2,764	3,525
Provision for income taxes	20,072	20,404	49,870	42,105
Net income attributable to Generac Holdings Inc.	$ 75,776	$ 39,435	$ 162,682	$ 76,901
Net income attributable to common shareholders per common share - basic: (in dollars per share)	$ 1.12	$ 0.64	$ 2.36	$ 1.25
Net income attributable to common shareholders per common share - diluted: (in dollars per share)	$ 1.11	$ 0.63	$ 2.34	$ 1.24
Comprehensive income attributable to Generac Holdings Inc.	$ 80,768	$ 42,939	$ 173,355	$ 90,867
Net income	$ 76,522	39,776	164,523	77,334
Deferred income taxes			17,218	25,336
Accounts receivable			(55,649)	(68,003)
Inventories			(99,957)	9,379
Other accrued liabilities			18,011	25,148
Net cash provided by operating activities			138,998	120,617
Proceeds from beneficial interests in securitization transactions			2,825	2,102
Net cash used in investing activities			$ (94,465)	(13,222)
Previously Reported [Member]
Accounts receivable, less allowance for doubtful accounts		320,626		320,626	280,002
Inventories		349,023		349,023	380,341
Other accrued liabilities		109,362		109,362	105,067
Deferred income taxes		49,528		49,528	43,789
Other long-term liabilities		69,556		69,556	76,995
Retained earnings		535,172		535,172	616,347
Accounts receivable		320,626		320,626	280,002
Inventories		349,023		349,023	380,341
Other accrued liabilities		109,362		109,362	105,067
Deferred income taxes		49,528		49,528	43,789
Other long-term liabilities		69,556		69,556	76,995
Retained earnings		535,172		535,172	616,347
Net sales		457,253		1,184,443
Costs of goods sold		299,784		782,028
Selling and service		44,402		127,702
Research and development		10,864		31,732
General and administrative		22,102		64,436
Other, net		1,570		3,105
Provision for income taxes		20,581		42,946
Net income attributable to Generac Holdings Inc.		$ 39,709		$ 78,211
Net income attributable to common shareholders per common share - basic: (in dollars per share)		$ 0.64		$ 1.27
Net income attributable to common shareholders per common share - diluted: (in dollars per share)		$ 0.64		$ 1.26
Comprehensive income attributable to Generac Holdings Inc.		$ 43,213		$ 92,177
Net income				78,644
Deferred income taxes				28,703
Accounts receivable				(70,108)
Inventories				16,738
Other accrued liabilities				17,319
Net cash provided by operating activities				122,719
Proceeds from beneficial interests in securitization transactions
Net cash used in investing activities				(15,324)
Restatement Adjustment [Member] | Accounting Standards Update 2014-09 [Member]
Accounts receivable, less allowance for doubtful accounts		(4,207)		(4,207)	(708)
Inventories		7,359		7,359	6,708
Other accrued liabilities		5,584		5,584	7,551
Deferred income taxes		(3,367)		(3,367)	(1,937)
Other long-term liabilities		6,178		6,178	5,898
Retained earnings		(5,243)		(5,243)	(5,513)
Accounts receivable		(4,207)		(4,207)	(708)
Inventories		7,359		7,359	6,708
Other accrued liabilities		5,584		5,584	7,551
Deferred income taxes		(3,367)		(3,367)	(1,937)
Other long-term liabilities		6,178		6,178	5,898
Retained earnings		(5,243)		(5,243)	$ (5,513)
Net sales		(1,414)		(3,244)
Costs of goods sold		(176)		1,219
Selling and service		(939)		(2,762)
Research and development		(14)		(42)
General and administrative		26		72
Other, net		140		420
Provision for income taxes		(177)		(841)
Net income attributable to Generac Holdings Inc.		$ (274)		$ (1,310)
Net income attributable to common shareholders per common share - basic: (in dollars per share)				$ (0.02)
Net income attributable to common shareholders per common share - diluted: (in dollars per share)		$ (0.01)		$ (0.02)
Comprehensive income attributable to Generac Holdings Inc.		$ (274)		$ (1,310)
Net income				(1,310)
Deferred income taxes				(3,367)
Accounts receivable				2,105
Inventories				(7,359)
Other accrued liabilities				7,829
Net cash provided by operating activities				(2,102)
Proceeds from beneficial interests in securitization transactions				2,102
Net cash used in investing activities				$ 2,102