Note 11 - Selected Quarterly Financial Data (Unaudited) - Selected Quarterly Financial Data (Unaudited) (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
Revenue (less agency commissions)	$ 233,609	$ 218,977	$ 226,681	$ 203,461	$ 237,619	$ 204,490	$ 196,633	$ 173,723	$ 882,728	$ 812,465	$ 597,356
Operating income	55,089	50,024	142,555	43,558	83,874	60,467	53,687	36,111	291,226	234,139	140,057
Net income	$ 165,570	$ 15,316	$ 70,561	$ 10,505	$ 35,834	$ (213)	$ 17,662	$ 8,990	$ 261,952	$ 62,273	$ 39,301
Net income (in dollars per share)	$ 2.15	$ 0.21	$ 0.98	$ 0.15	$ 0.50	$ 0	$ 0.25	$ 0.13	$ 3.59	$ 0.87	$ 0.58
Diluted net income per share (in dollars per share)	$ 2.13	$ 0.21	$ 0.97	$ 0.14	$ 0.49	$ 0	$ 0.24	$ 0.12	$ 3.55	$ 0.86	$ 0.57