Note 12 - 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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue (less agency commissions)	$ 237,619	$ 204,490	$ 196,633	$ 173,723	$ 169,487	$ 151,102	$ 143,464	$ 133,303	$ 812,465	$ 597,356	$ 508,134
Operating income	83,874	60,467	53,687	36,111	43,897	29,344	38,758	28,058	234,139	140,057	153,773
Net income	$ 35,834	$ (213)	$ 17,662	$ 8,990	$ 14,987	$ 6,609	$ 12,110	$ 5,595	$ 62,273	$ 39,301	$ 48,061
Net income (in dollars per share)	$ 0.50		$ 0.25	$ 0.13	$ 0.21	$ 0.09	$ 0.17	$ 0.10	$ 0.87	$ 0.58	$ 0.83
Diluted net income per share (in dollars per share)	$ 0.49		$ 0.24	$ 0.12	$ 0.21	$ 0.09	$ 0.17	$ 0.10	$ 0.86	$ 0.57	$ 0.82