Note 17 - Unaudited Quarterly Financial Data - Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues	$ 1,208,103	$ 1,257,459	$ 1,203,523	$ 1,044,797	$ 1,021,271	$ 1,096,041	$ 1,026,462	$ 1,070,840	$ 4,713,882	$ 4,214,614	$ 4,979,733
Gross profit	212,909	220,048	209,072	187,907	172,948	181,316	180,478	183,270	829,936	718,012	784,947
Operating income	38,017	48,436	37,407	24,849	23,387	27,588	21,572	8,181	148,709	80,728	121,276
Income before income taxes	34,423	45,335	34,583	22,058	20,395	24,303	17,809	3,942	136,399	66,449	107,803
Net income	$ 105,867	$ 29,784	$ 21,999	$ 14,479	$ 12,490	$ 14,880	$ 10,817	$ 2,395	$ 172,129	$ 40,582	$ 66,053
Earnings per common share:
Basic (in dollars per share)	$ 2.65	$ 0.75	$ 0.55	$ 0.37	$ 0.32	$ 0.38	$ 0.27	$ 0.06	$ 4.34	$ 1.02	$ 1.64
Diluted (in dollars per share)	$ 2.54	$ 0.72	$ 0.54	$ 0.36	$ 0.31	$ 0.37	$ 0.27	$ 0.06	$ 4.20	$ 1	$ 1.61
Revenues	$ 1,208,103	$ 1,257,459	$ 1,203,523	$ 1,044,797	$ 1,021,271	$ 1,096,041	$ 1,026,462	$ 1,070,840	$ 4,713,882	$ 4,214,614	$ 4,979,733
Gross profit	212,909	220,048	209,072	187,907	172,948	181,316	180,478	183,270	829,936	718,012	784,947
Operating income	38,017	48,436	37,407	24,849	23,387	27,588	21,572	8,181	148,709	80,728	121,276
Income before income taxes	34,423	45,335	34,583	22,058	20,395	24,303	17,809	3,942	136,399	66,449	107,803
Net income	$ 105,867	$ 29,784	$ 21,999	$ 14,479	$ 12,490	$ 14,880	$ 10,817	$ 2,395	$ 172,129	$ 40,582	$ 66,053