Note 12 - Selected Quarterly Financial Data (Unaudited) - Schedule of Selected Quarterly Financial Data (Unaudited) (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
Revenue (less agency commissions)	$ 328,220	$ 279,310	$ 250,344	$ 226,258	$ 233,609	$ 218,977	$ 226,681	$ 203,461	$ 1,084,132	$ 882,728	$ 812,465
Operating income	150,644	108,402	79,690	50,035	54,965	49,900	142,394	43,472	388,771	290,731	234,304
Net income	$ 88,267	$ 61,886	$ 40,705	$ 19,945	$ 165,570	$ 15,316	$ 70,561	$ 10,505	$ 210,803	$ 261,952	$ 62,273
Net income (in dollars per share)	$ 1.01	$ 0.71	$ 0.46	$ 0.22	$ 2.15	$ 0.21	$ 0.98	$ 0.15	$ 2.39	$ 3.59	$ 0.87
Diluted net income per share (in dollars per share)	$ 1	$ 0.70	$ 0.46	$ 0.22	$ 2.13	$ 0.21	$ 0.97	$ 0.14	$ 2.37	$ 3.55	$ 0.86