Note 6 - Segment Reporting - Net Sales by Products and Services (Details) - USD ($) $ 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
Domestic [Member]
Net sales									1,580,325	1,303,506	1,176,849
International [Member]
Net sales									443,139	375,867	270,894
Residential Power Products [Member]
Net sales									1,042,739	870,490	769,176
Residential Power Products [Member] | Domestic [Member]
Net sales									990,631	815,125	730,288
Residential Power Products [Member] | International [Member]
Net sales									52,108	55,365	38,888
Commercial and Industrial Power Products [Member]
Net sales									820,270	684,353	558,468
Commercial and Industrial Power Products [Member] | Domestic [Member]
Net sales									464,066	385,575	346,094
Commercial and Industrial Power Products [Member] | International [Member]
Net sales									356,204	298,778	212,374
Other Products and Services [Member]
Net sales									160,455	124,530	120,099
Other Products and Services [Member] | Domestic [Member]
Net sales									125,628	102,806	100,467
Other Products and Services [Member] | International [Member]
Net sales									$ 34,827	$ 21,724	$ 19,632