Note 16 - Unaudited Quarterly Financial Data - Quarterly Financial Data (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
Total revenue	$ 1,540,442	$ 1,376,136	$ 1,348,831	$ 1,240,781	$ 1,208,103	$ 1,257,459	$ 1,203,523	$ 1,044,797	$ 5,506,190	$ 4,713,882	$ 4,214,614
Gross profit	258,840	249,057	243,377	226,995	212,909	220,048	209,072	187,907	978,269	829,936	718,012
Operating income	68,387	58,649	43,426	32,389	38,017	48,436	37,407	24,849	202,851	148,709	80,728
Income before income taxes	61,973	54,181	38,932	28,083	34,423	45,335	34,583	22,058	183,169	136,399	66,449
Net income	$ 46,969	$ 41,665	$ 29,389	$ 21,039	$ 105,867	$ 29,784	$ 21,999	$ 14,479	$ 139,062	$ 172,129	$ 40,582
Basic (in dollars per share)	$ 1.22	$ 1.06	$ 0.75	$ 0.53	$ 2.65	$ 0.75	$ 0.55	$ 0.37	$ 3.55	$ 4.34	$ 1.02
Diluted (in dollars per share)	$ 1.20	$ 1.03	$ 0.72	$ 0.51	$ 2.54	$ 0.72	$ 0.54	$ 0.36	$ 3.45	$ 4.20	$ 1