Note 16 - Unaudited Quarterly Financial Data - Quarterly Financial Data (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
Total revenue	$ 1,317,704	$ 1,599,265	$ 1,544,561	$ 1,348,317	$ 1,540,442	$ 1,376,136	$ 1,348,831	$ 1,240,781	$ 5,809,847	$ 5,506,190	$ 4,713,882
Gross profit	234,438	264,768	269,506	256,916	258,840	249,057	243,377	226,995	1,025,628	978,269	829,936
Operating income	39,423	58,323	61,792	56,867	68,387	58,649	43,426	32,389	216,405	202,851	148,709
Income before income taxes	33,345	52,210	54,410	49,558	61,973	54,181	38,932	28,083	189,523	183,169	136,399
Net income	$ 23,754	$ 39,104	$ 41,621	$ 37,104	$ 46,969	$ 41,665	$ 29,389	$ 21,039	$ 141,583	$ 139,062	$ 172,129
Basic (in dollars per share)	$ 0.65	$ 1.07	$ 1.13	$ 1.01	$ 1.22	$ 1.06	$ 0.75	$ 0.53	$ 3.86	$ 3.55	$ 4.34
Diluted (in dollars per share)	$ 0.64	$ 1.05	$ 1.10	$ 0.98	$ 1.20	$ 1.03	$ 0.72	$ 0.51	$ 3.77	$ 3.45	$ 4.20