Note 12 - Selected Quarterly Financial Data (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue (less agency commissions)	$ 169,487,000	$ 151,102,000	$ 143,464,000	$ 133,303,000	$ 177,886,000	$ 131,702,000	$ 107,249,000	$ 91,297,000	$ 597,356,000	$ 508,134,000	$ 346,298,000
Operating income	43,897,000	29,344,000	38,758,000	28,058,000	72,021,000	41,156,000	23,186,000	17,410,000	140,057,000	153,773,000	83,880,000
Net income	$ 14,987,000	$ 6,609,000	$ 12,110,000	$ 5,595,000	$ 31,253,000	$ 13,940,000	$ 1,591,000	$ 1,277,000	$ 39,301,000	$ 48,061,000	$ 18,288,000
Net income (in dollars per share)	$ 0.21	$ 0.09	$ 0.17	$ 0.10	$ 0.54	$ 0.24	$ 0.03	$ 0.02	$ 0.58	$ 0.83	$ 0.32
Diluted net income per share (in dollars per share)	$ 0.21	$ 0.09	$ 0.17	$ 0.10	$ 0.53	$ 0.24	$ 0.03	$ 0.02	$ 0.57	$ 0.82	$ 0.32